OMAPDSS: Add Video PLLs for DRA7xx

DRA7xx SoCs have one (DRA72x) or two (DRA74x) video PLLs. They are
basically the same as DSI PLLs on OMAPs, but without the rest of the DSI
hardware. The video PLLs also require some configuration via the CONTROL
module.

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