Low power mode for Portenta system

Hi, I want to ask if the low-power mode of Portenta H7 is supported using Arduino code. I read through the repo and found that the [enterLowPower ](https://github.com/arduino/ArduinoCore-mbed/blob/933a3e143adb9d0dca13515800ad02261a7675eb/libraries/STM32H747_System/src/Portenta_System.cpp)function of the Portenta system still needs to be implemented. I am wondering if there's any plan to implement it or any example I could start with.

Start with libraries/STM32H747_System/src/Portenta_System.cpp and inspect the enterLowPower function referenced in the issue. Clarify the expected Portenta H7 low-power behavior and implementation requirements before proceeding; done means the function is implemented and the requested low-power mode is usable from Arduino code.

Written by the indexing model from the issue text.