fbdev: sh_mobile_hdmi: improve support for more video modes

Configure pre-programmed VIC modes for VGA (640x480@60) and 720x576@50
modes, change PHY mode selection to be based on pixel clock frequency.

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