fbdev: sh_mobile_lcdc: use the standard CEA-861 720p timing

fbdev: sh_mobile_lcdc: use the standard CEA-861 720p timing

sh_mobile_lcdcfb.c has a hard-coded 720p video mode, used as default, if none
is explicitly specified by the platform. Adjust its timing to match the CEA
standard. Also add an explicit refresh rate value, which is needed, when used
with HDMI, to be able to recognise the default 720p mode as a pre-programmed
VIC #4.

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