ARM: dts: stm32: add pin map for LTDC on stm32f7

Add pin configurations for using LTDC (LCD-tft Display Controller) on
stm32f746-disco board.

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>