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 <aurelien@aurel32.net>