OMAPDSS: add of helpers

Add helpers to get ports and endpoints from DT data.

These helpers parse v4l2 style ports and endpoints, but compared to the
v4l2 helpers, these allow iterating ports and endpoints separately.

This is a temporary solution until we get generic code to parse the
ports and endpoints.

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