media: rcar-vin: Add support for V4L2_FIELD_ALTERNATE

The hardware is capable to passing V4L2_FIELD_ALTERNATE to user-space.
Allow users to request this field format but still default to using the
hardware interlacer if alternating is not explicitly requested.

Before this change a sensor providing data using alternate would be
always combined to an interlaced frame. After this change the user can
request to receive frames as alternate if the sensor provides it.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>