platform: Fixup nrf54l targets lc10a and lv10a

[sigvartmh]

Change the watchdog and uart configs which are wrong.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Updates the nRF54L non-secure PAL configuration to use a single UART and watchdog instance while clarifying how the watchdog instance is selected in the driver.

Changes:

• Reduce UART configuration from an out-of-range placeholder (20) to a single instance (0)
• Reduce watchdog configuration from a placeholder instance (4) to a single instance (0) and update explanatory comments
• Update platform-level macro mappings to the new UART/watchdog instance macros

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[avinaw01-arm]

Hi @sigvartmh
Thanks for raising the P.R.

I noticed that PLATFORM_WD_TIMEOUT_IN_MICRO_SEC_MEDIUM and PLATFORM_WD_TIMEOUT_IN_MICRO_SEC_HIGH are both assigned the value of PLATFORM_WD_TIMEOUT_IN_MICRO_SEC_LOW in your config file. Even if these values are currently unused on your platform, would it make sense to assign the respective timeout values to these platform macros for consistency and correctness?

Could you please take a look and let us know your thoughts? Apart from this, the rest of the P.R. looks good to me and should be fine to merge. Thank you.