arch/arm/am67: Add EPWM0/EPWM1 PWM lower-half driver

[gemstonedevteam]

Opening as draft since this depends on #20136 will mark ready once that lands.

Summary

• Adds a PWM lower-half driver for the AM67 EPWM instances, covering both
  output channels (EPWMxA and EPWMxB) of EPWM0 and EPWM1.
• The time-base is clocked from the EPWM functional clock, period in TBPRD
  and duty in CMPA/CMPB, with the action-qualifier configured for standard
  edge-aligned output: set on zero, clear on compare.
• The CTRL_MMR EPWM clock enables are unlocked once during board bring-up
  before any instance is configured.
• t3-gem-o1 registers EPWM0 as /dev/pwm0 and EPWM1 as /dev/pwm1, with
  CONFIG_PWM_NCHANNELS=2 so both channels are reachable through one device.

Impact

• Is new feature added? YES — PWM support for AM67.
• Impact on user? NO — new driver, opt-in by Kconfig.
• Impact on build? NO.
• Impact on hardware? YES — arch/arm/am67 and board t3-gem-o1.
• Impact on documentation? YES — provided in this PR (second commit).
• Impact on security? NO.
• Impact on compatibility? NO — additive.
• Anything else? Depends on PRs 3–4.

Testing

Build Host(s): Pardus GNU/Linux 25 (Debian 13 base), x86_64, host gcc 14
Toolchain:     arm-none-eabi-gcc 14.2.1 20241119 (15:14.2.rel1-1)
Target:        arm / TI AM67 (J722S) Cortex-R5F, board t3-gem-o1:nsh
nuttx-apps:    5a7ab4200
Loaded on the main-domain R5F by Linux remoteproc; console on UART-MAIN1.

Testing logs before change: no PWM support on this arch.

The version string in the logs is from the tested build; the commits were
GPG-signed afterwards, which rewrites hashes. The code is unchanged.

Testing logs after change:

make distclean && ./tools/configure.sh t3-gem-o1:nsh && make -j
  -> nuttx ELF 541496 bytes, 0 compiler warnings

Console:
nsh> pwm -p /dev/pwm0 -f 1000 -d 50 -t 1
pwm_main: starting output with frequency: 1000 channel: 1 duty: 00007fff channel: 2 duty: 00007fff
pwm_main: stopping output
nsh> pwm -p /dev/pwm1 -f 1000 -d 50 -t 1
pwm_main: starting output with frequency: 1000 channel: 1 duty: 00007fff channel: 2 duty: 00007fff
pwm_main: stopping output
nsh>

Test plan result:
console_alive            PASS  prompt responsive
version                  PASS  NuttX version 0.0.0 d4055f59a2 Sep 10 2026 13:48:45 t3-gem-o1:nsh
procfs                   PASS  procfs readable
tasks                    PASS  2 task lines
dev_nodes                PASS  all expected nodes present
pwm_pwm0                 PASS  /dev/pwm0 1 kHz / 50% for 1 s
pwm_pwm1                 PASS  /dev/pwm1 1 kHz / 50% for 1 s

PASS=7

Measured time base, captured 2026-09-11. The log above only shows the ioctl
returning; this reads the registers back from the A53 over /dev/mem while
NuttX is driving the output.

NuttX:  nsh> pwm -p /dev/pwm0 -f 1000 -d 50 -t 25 &

A53, while that is running (32-bit reads: the peripheral returns 0 for an
8-bit access, so the 16-bit registers are taken as halves of a word):

  0x23000000 = 67640        -> TBCTL = 0x0838
  0x23000008 = 4095981386   -> TBCNT = 46922, TBPRD = 62499
  0x23000010 = 2047934464   -> CMPA  = 31249

  TBCTL 0x0838: HSPCLKDIV field 0 -> divider 1, CLKDIV field 2 -> divider 4
  frequency = 250 MHz / (1 * 4 * (62499 + 1)) = 1000.00 Hz
  duty      = 31249 / 62500                   = 49.998 %

  TBCNT sampled twice 0.4 s apart: 62272 -> 61319, so the time base is
  clocked and running, not merely loaded.

/dev/pwm1 at 0x23010000 reads identically: TBCTL 0x0838, TBPRD 62499,
CMPA 31249 -> 1000.00 Hz, 49.998 %, TBCNT 46456 -> 8224.

CTRLMMR_EPWM_TB_CLKEN (0x00104130) = 0x5, so the EPWM0 clock gate the
driver opens during bring-up is in fact open.

PR verification Self-Check

• This PR introduces only one functional change.
• I have updated all required description fields above.
• My PR adheres to Contributing Guidelines and Documentation.
• My PR is still work in progress (not ready for review).
• My PR is ready for review and can be safely merged into a codebase.

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb