fbdev: sh_mobile_lcdc: make platform videomode table optional

Add a default 720p mode to the sh_mobile_lcdc driver to be used, when no
videomode is specified in the platform data. This can be used, e.g., with HDMI.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>