dts: stm32l4: add flash write-block-size
dts: stm32l4: add flash write-block-size

This makes the SoC flash compatible with the common nonvolatile flash
YAML schema, and provides a write alignment. It mirrors work done on
nRF chips for the DFU subsystem.

Signed-off-by: Marti Bolivar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@linaro.org>

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