drm/gma500/mrst: Add SDVO clock calculation

We start off by adding SDVO limits and converting all limits to the
generic gma_limit_t stuct. Then we separate clock calculations for
LVDS and SDVO. This will be cleaned up later but keep it simple for now.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>