CatenaStm32L0Rtc::SleepForAlarm() calls __HAL_PWR_CLEAR_FLAG w/o clocking

[terrillmoore]

CatenaStm32L0Rtc::SleepForAlarm() calls __HAL_PWR_CLEAR_FLAG(PWR_FLAG_WU), but doesn't ensure that the clock to the PWR register is turned on. In fact, by default, it's off. This makes this call useless. There is a new API in future BSPs, but testing indicates that this API is not needed -- the system still sleeps.

E.g.

Catena-Arduino-Platform/src/lib/stm32/stm32l0/CatenaStm32L0Rtc.cpp

Lines 381 to 382 in d0a03aa

	__HAL_PWR_CLEAR_FLAG(PWR_FLAG_WU);
	HAL_PWR_EnterSTOPMode(

Solution: remove calls.