[media] v4l2: Add polarity flag definitions for the parallel bus FIELD signal

FIELD signal is used for indicating frame field type to the frame grabber
in interlaced scan mode, as specified in ITU-R BT.601 standard.
In normal operation mode FIELD = 0 selects Field1 (odd) and FIELD = 1
selects Field2 (even). When the FIELD signal is inverted it's the other
way around.

Add corresponding flags for configuring the FIELD signal polarity,
V4L2_MBUS_FIELD_EVEN_HIGH for the standard (non-inverted) case and
V4L2_MBUS_FIELD_EVEN_LOW for inverted case.

Also add a comment about usage of V4L2_MBUS_[HV]SYNC* flags for
the hardware that uses [HV]REF signals.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>