lib: stm32wba: use reference count for backup domain accesses

Add LINKLAYER_PLAT_EnableBackupDomainAccess()  and
LINKLAYER_PLAT_DisableBackupDomainAccess() to use Zephyr resources
that use a reference counter for access requests, for enabling
and disabling access the BackupDomain resources.

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