ARM: omap2.dtsi: add omapdss information

Add DT data for OMAP2 display subsystem, which contains the following
blocks:

dss - the wrapper/glue for the display modules
dispc - display controller
rfbi - MIPI DBI encoder
venc - analog TV encoder

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