drivers: counter: stm32: Fix RTC write access requests

Add missing backup domain access requests in STM32 RTC counter
driver that are needed to modify RTC registers.

Remove backup domain access requests in rtc_stm32_read() since this
function only reads RTC registers and backup domain access protection
protects RTC registers against write accesses only.

Fixes issue 92511.

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