OMAPDSS: add dedicated fck PLL support

This patch adds support for SoCs that have a dedicated DSS PLL used for
DSS function clock.

If there is no dss parent clock defined, it is presumed that the
functionl clock rate can be set (almost) freely. The code calculates the
highest allowed fck rate, which when divided with some integer gives the
required pck.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>