drivers: flash: stm32wba: Use STM32WBA Flash manager for RF coexistence

When Bluetooth is enabled, it is required to arbitrate flash accesses
between RF and write accesses (for user activity).
A dedicated flash manager is provided as part of STM32WBA BLE lib.

Implement a dedicated driver using FM Apis to handle RF activity.

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