soc: st: Add support for STOP3 on STM32U5

LPTIM is not available in STOP3 mode, so RTC needs to be used instead.
This code usese similar approach as STM32WBAx for suspend to ram.
The STOP3 is disabled by default in device tree.

Signed-off-by: Adam Berlinger <adam.berlinger@st.com>
7 files changed