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.