omap3_dss: add optional framebuffer

omap3_dss: add optional framebuffer

The beagle board uses the background color to show an orange screen during
startup. This patch adds the ability to add a frame buffer, with the
intention not to break the beagle board use case (I don't have one).
videomodes.c is not used. Scrolling depends on this patch:

http://patchwork.ozlabs.org/patch/155662/

cc: trini@ti.com
cc: s-paulraj@ti.com
cc: khasim@ti.com
Signed-off-by: Jeroen Hofstee <jhofstee@victronenergy.com>