stm32/powerctrl: Save and restore EWUP state when configuring standby.
stm32/powerctrl: Save and restore EWUP state when configuring standby.

This allows the user to enable wake-up sources using the EWUP bits, on F7
MCUs.

Disabling the wake-up sources while clearing the wake-up flags follows the
reference manual and ST examples.

bvernoux/micropython,henriknelson/micropython,bvernoux/micropython,henriknelson/micropython,bvernoux/micropython,henriknelson/micropython,henriknelson/micropython,adafruit/circuitpython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,henriknelson/micropython