media: v4l2_core: Add p_area to struct v4l2_ext_control

Allow accessing V4L2_CTRL_TYPE_AREA controls without any casting.

Signed-off-by: Ricardo Ribalda Delgado <ribalda@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>