ARM: dts: am4372.dtsi: add DSS information

Add DT data for the display subsystem for AM4372. The DSS on AM4372 is
basically OMAP3's DSS, without DSI and VENC blocks.

Signed-off-by: Sathya Prakash M R <sathyap@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>