arm/dts: dra7.dtsi: add DSS support

DRA7xxx contains a very similar DSS to OMAP5. The main differences are:

* no DSI or RFBI support.
* 1 or 2 dedicated video PLLs.
* need to do additional configuration to the DRA7 CONTROL module.

DRA72xx has only one video PLL, and DRA74xx has two.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: devicetree@vger.kernel.org
Acked-by: Tony Lindgren <tony@atomide.com>