dts: stm32f7: move flash0 node below flash-controller
dts: stm32f7: move flash0 node below flash-controller

Due to a bad timing in merging PR #10744 and PR #12083, the flash0 node
in stm32f756Xg.dtsi ended-up at the root of the device tree, while the
flash0 node it is trying to override in stm32f7.dtsi is now under
/soc/flash-controller.

This patch fixes that by moving it at the right location.

Signed-off-by: Aurelien Jarno <bceb368e7f2cb351af47298f32034f0587bbe4a6@aurel32.net>

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