ARM: dts: stm32: support display on stm32f746-disco board

The patch applies the changes from Linux commit 10a970bc3ebfa ("ARM: dts:
stm32: support display on stm32f746-disco board") and removes the same
settings from stm32f746-disco-u-boot.dtsi.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>