ARM: dts: am57xx: sbc-am57x: add LCD support

Startek-kd050c 800x480 LCD panel timings are described in
compulab-sb-som.dtsi.

Add appropriate DT endpoints to connect DPI output and LCD.

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>