drivers: clock_stm32: add APB3 support for STM32WL

STM32WL series have an extra APB3 bus with the SUBGHZSPI device on it.
Add the relevant code to enable and disable that clock, and to obtain
the actual clock rate. This is enough to run the STM32 SPI driver
against it.

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>