soc: st: stm32: reference counter for Backup SRAM accesses

Use recently added SoCs functions to request access to SoC backup
domain resources These function use a reference counter to track these
request and ensure the resources are accessible as long as at least a
consumer requires access.

Fixes issue 90942.

Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>