V4L/DVB: v4l2_subdev: Add s_io_pin_config to v4l2_subdev_core_ops

Add a method to v4l2_sudev_core_ops to allow bridge drivers to
manage what signal pads/functions are routed out to multiplexed IO pins on a
pin by pin basis.  The interface also allows specifying initial output settings
for pins and disabling an IO pin altogether.

Signed-off-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>