[media] videodev2.h: add V4L2_FIELD_HAS_T_OR_B macro

Add a macro to test if the field consists of a single top
or bottom field. Anyone who needs to work with fields as opposed to
frame will need this.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>