drivers: flash_stm32: enable flash_stm32l4x for STM32WL series

The flash_stm32l4x driver seems to work out of the box on the WL series.

This just adds the necessary config changes to let the driver build and
run when SOC_SERIES_STM32WLX is selected.

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