drivers: flash: stm32: get write block size from device tree
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 <de4d967452b83bb0e37525652734e7d9163b6a4c@linaro.org>

zephyriot/zephyr,Vudentz/zephyr,zephyriot/zephyr,explora26/zephyr,explora26/zephyr,kraj/zephyr,zephyriot/zephyr,ldts/zephyr,ldts/zephyr,mbolivar/zephyr,kraj/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,ldts/zephyr,zephyriot/zephyr,explora26/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,punitvara/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,mbolivar/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,galak/zephyr,kraj/zephyr,kraj/zephyr,Vudentz/zephyr,kraj/zephyr,ldts/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,nashif/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,punitvara/zephyr,Vudentz/zephyr,ldts/zephyr,Vudentz/zephyr,zephyriot/zephyr,finikorg/zephyr,punitvara/zephyr,explora26/zephyr,nashif/zephyr,finikorg/zephyr,explora26/zephyr