board: stm32f469-disco: add support to display

Add support to Orise Tech OTM8009A display on stm32f469-disco board.

It was necessary to retrieve the framebuffer address from the device tree
because the address returned by the video-uclass driver pointed to a memory
area that was not usable.

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