ARM: dts: stm32: support MIPI DSI on stm32f469-disco board

Unlike Linux, the DSI driver requires the LTDC clock to be properly
probed. Hence, the changes made to the DSI node.

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