ARM: dts: keystone-k2g: Add DSS node

Add DT node for DSS. K2G has DSS version 6. Keystone family DSS are
somewhat different compered to the old OMAP family DSSes and the
binding looks different too.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>