OMAP: DSS2: move dss device clock configuration

Clock configuration was defined inside dssdev.phy.dsi struct. The clock
config doesn't really belong there, and so it's moved to dssdev.clock
struct.

Now the explicit clock configuration could also be used for other
interfaces than DSI, although there's no support for it currently.

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