[media] v4l2-mc.h Add pads for audio and video IF-PLL decoders

The audio and video IF-PLL decoders have one sink and one source
PAD. Add macro names for those pads and describe what kind of
signals are represented at such pads.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>