drivers: bbram: stm32-bbram: port to stm32wl

On STM32WL, the backup memory is defined as part of the TAMP peripheral.
This seems to be a deviation from the stm32 family where this memory is
defined as part of the RTC.

The STM32WL reference manual shows that tamp_pclk is connected to
rtc_pclk. This means that the clock required to run the TAMP peripheral
is the same as the RTC's. A quick port of BBRAM on STM32WL is achieved
by instanciating the bbram device as a child of the RTC and by modifying
the address offset to the first backup register from the rtc base
address.

Signed-off-by: Adrien Bruant <adrien.bruant@aalberts-hfc.com>