media: omap3isp: Rework OF endpoint parsing

Rework OF endpoint parsing for the omap3isp driver. This does add some
lines of code. The benefits are still clear:

- the great complication related to callbacks in endpoint parsing is gone;
  instead endpoints are obtained port by port and

- endpoints may now have a default bus configuration which was not
  possible while using callbacks. This driver does not benefit from that
  feature, but as the omap3isp is one of the exemplary drivers, this works
  as an example for driver developers.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Tested-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>