OMAPDSS: DPI: DRA7xx support

Add support for DRA7xx DPI output.

DRA7xx has three DPI outputs, each of which gets its input from a DISPC
channel. However, DRA72x has only one video PLL, and DRA74x has two
video PLLs. In both cases the video PLLs need to be shared between
multiple outputs. The driver doesn't handle this at the moment.

Also, DRA7xx requires configuring CONTROL module bits to route the clock
from the PLL to the used DISPC channel.

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