Use more efficient mode for STM32L4XX stop mode

[angelnu]

Summary

This PR fixes/implements the following bugs/features

• Feature 1 - save power in stop mode for STM32L4XX devices
• Feature 2 - Use MSI as wake up clock when available
• Breaking changes

Feature 1
Currently the low power library uses HAL_PWR_EnterSTOPMode for all devices. This does not leverage the stop2 mode which is available in the STM32L4XX. This results in 4,6 uA instead of 1,35 uA in my nucleo_l412kb.

Stop1 is only required when the UART device needs to wake up. So when not used the patch calls HAL_PWREx_EnterSTOP2Mode directly.

Feature 2
When available use the MSI as wake up clock. This avoids starting the HSI which generates a short spike of ca 300 uA until the SystemClock_ConfigFromStop is called.

Validation

• Ensure CI build is passed.
• Demonstrate the code is solid. [e.g. Provide a sketch]

Not adding a new sketch since no external APIs are changed by this patch.

Code formatting

• Ensure AStyle check is passed thanks CI

Closing issues
NA

[fpistm]

Hi @angelnu
as you know I'm currently working on the SubSeconds managements and I'm studying to move the low power driver to the Low Power library. Anyway, I have to study the impact of the mode you propose.

[angelnu]

Jep, I know (looking forward to it) :-)

This patch is independent from the sleep time. I made it specific for the STM32L4 since this is the only device I have that supports STOP2. It would be nice if there would be a define for all platforms with STOP2...

I hope I can get the style check to agree so the patch is ready. If you have any other feedback you would like to get here please let me know.