drivers: counter: Add support for stm32wba devices

Implement RTC support in counter driver for STM32WBA devices.

Changes are made according to the following specificities:
- Similarly to STM32U5, it is not connected to EXTI.
- On this series, there is no bit in BCDR register to enable RTC. Enabling
RTC is done directly via the RCC APB register bit

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>