soc: stm23u5: Provide power implementation

Provide power modes implementation for u5 socs.
For now STOP3 mode is not implemented as this mode is not
compatible with LPTIM activation and hence cannot be used
as a workable suspend to idle state using LPTIM as kernel
tick source.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>