OMAP: DSS2: Introduce dss_features files

Add dss_features.c and dss_features.h for the dss_features framework.
This framework will be used to move all cpu_is_xxx() and similar calls
to a single place.

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