OMAPDSS: Add support for LCD3 channel

OMAP5 Display Subsystem (DSS) architecture comes with a additional LCD3 channel
with its own dedicated overlay manager. The current patch adds LCD3 channel and
basic register support for LCD3 channel. It adds register addresses for various
Display Controller (DISPC) registers like DISPC_DEFAULT_COLOR, DISPC_TIMING_H,
DISPC_DIVISORo, etc.

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