drivers/flash: stm32l4: Prepare for unaligned accesses in flash writes

On STM32L4, flash writes operations are performed 8 bytes at a time.
Though, it is possible that *data in flash_write functions is not
aligned. To avoid issues, use UNALIGNED_GET macro to access *data.

Issue has been detected using settings subsystem on STM32WB, which has
same 8bytes write block size. The patch is extended to STM32L4 series
for same reason.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>