[media] v4l2-mc.h: move tuner PAD definitions to this new header

The customer PC hardware can be shipped with lots of different
configurations, as vendors use to replace some of the chips on
their hardware along the time. All drivers that support such
devices are prepared to handle the hardware differences, using
their own auto-probing logic.

They do it in a way that number of inputs and outputs for a given
hardware type doesn't change.

Now that we're adding media controller capabilities to those drivers,
we need to standardize the number of inputs and outputs for each
hardware type, as we want to have a generic function at the V4L2
core that would create the links for the entities that are expected
on such hardware.

Such standard is already there for tuners, but tuner.h is not the
best place to store such data, as we'll need to add definitions also
for analog TV demodulators.

Also, we'll need a place to put a set of MC handling functions. So,
let's create a v4l2-mc.h to store such kind of definitions.

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