STM32L4: support UART5 for debug output

[xarantolus]

This MR basically just resurrects some code I've written quite some time ago. It's also integrated with the config system, so a user can just select UART5 and it'll work.

Tested output using the hello-world example on cave and that works, but didn't really test the UART5_RX part, but that should be the correct pin. But tbh we probably don't care.

One thing that would be a bit nicer is if we did the comparison of huart->Instance at compile time / only compiled in the relevant code, but didn't really get that working nicely with preprocessor macros - any opinion on that?

[Copilot]

Pull request overview

Adds STM32L4 debug output support for selecting UART5 in addition to the existing LPUART1 path, wiring up the necessary RCC clock and GPIO AF configuration in the STM32L4xx UART interface.

Changes:

• Extend debug.uart configuration option to allow selecting UART5.
• Add UART5-specific MSP init (clock selection + GPIOC/ GPIOD pin configuration) in the STM32L4xx UART interface.
• Update UART interface includes to pull in additional STM32/HAL headers.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
options.toml	Expands the debug UART selection to include UART5.
machine/arm/stm32l4xx/interface/uart.c	Implements UART5 clock + GPIO init path for debug UART output.

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[github-actions]

LCOV of commit 0561a48 during Osiris CI #312

This pull request changes total coverage 0.00% (14.04% -> 14.04%) for this diff

lcov: WARNING: lcov: WARNING: RC option 'lcov_branch_coverage' is deprecated.  Consider using 'branch_coverage. instead.  (Backward-compatible support will be removed in the future
Summary coverage rate:
  lines......: 14.0% (232 of 1653 lines)
  functions..: 16.7% (39 of 233 functions)
  branches...: no data found

Files changed coverage rate: n/a