[media] au8522: build ATV/DTV demodulators as separate modules

au8522_dig.o and au8522_decoder.o function independentantly
of each other, each for a different hardware function using
a different software subsystem api, each with its own set
of subsystem module dependencies.

Since these drivers do not depend on each other, and it is
in fact possible to build hardware designs using one function
and not the other, lets split this module into two, allowing
system integrators to enable the hardware without dragging in
undesired dependencies.

Signed-off-by: Michael Krufky <mkrufky@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>