Enable  getting of subdevs from DT ports and endpoints.

The _get_pdata() function was larely inspired by (i.e. stolen from)
am437x-vpfe.c
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>