Doc/DT: Add DT binding documentation for OMAP DSS

Add device tree bindings for OMAP Display Subsystem for the following
SoCs: OMAP2, OMAP3, OMAP4.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Archit Taneja <archit@ti.com>