drivers: flash: stm32: get write block size from device tree

In flash driver init, write_block_size could be initialized
with FLASH_WRITE_BLOCK_SIZE which is generated from device
tree parsing (dts property: "write-block-size")
Fixes #5305
If not defined, generate a compilation error.

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