The HW can capture both ODD and EVEN fields in separate buffers so it's
possible to support V4L2_FIELD_ALTERNATE. This patch add support for
this mode.

At probe time and when S_STD is called the driver will default to use
V4L2_FIELD_INTERLACED if the subdevice reports V4L2_FIELD_ALTERNATE. The
driver will only change the field type if the subdevice implements
G_STD, if not it will keep the default at V4L2_FIELD_ALTERNATE.

The user can always explicitly ask for V4L2_FIELD_ALTERNATE in S_FMT and
the driver will use that field format.
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>