drivers: flash: stm32l5: use write-block-size when writing

STM32L5 have a write block size of 8, but STM32U5 and STM32H5 have a
write block size of 16. Convert write_dword() and
flash_stm32_write_range() to write write-block-size data at a time.

Fixes #60724

Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>