ARM: dts: stm32: add ltdc support on stm32f746 MCU

Add LTDC (Lcd-tft Display Controller) support.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Raphaël Gallais-Pou <raphael.gallais-pou@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>