ARM: OMAP2+: add omapdss_init_of()

The OMAP display architecture requires a bunch of platform devices which
are not created via .dts (for now). We also need to pass a few function
pointers and the DSS hardware version from the arch code to omapdss
driver.

This patch adds omapdss_init_of() function, called from board-generic at
init time, which handles those tasks.

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