OMAPDSS: Add DT support to DSS

Add DT support for DSS. Contrary to the non-DT version, the DSS in DT
mode contains DPI and SDI outputs, which better reflects the hardware.
The non-DT code will be removed after all boards have been converted to
DT, so there's no need to change the non-DT code to act the same way.

The code for DPI and SDI needs to be refined later to make it possible
to add multiple DPI ports. For now, handling just a single DPI port is
enough for all the boards.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Archit Taneja <archit@ti.com>