drivers: flash: stm32f4: Flush caches after erase
drivers: flash: stm32f4: Flush caches after erase

This implement the same flush cache functionality already present in the
other stm32 series flash drivers, used to avoid bus errors when writing
big chunks of data to the flash.

Signed-off-by: Fabio Baltieri <161d407f4dc681bfd4aedb33b8273b2aafd727bc@gmail.com>

finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr