[media] media.h: add support for IF-PLL video/sound decoder

Very old hardware may have an analog stage tuner. Those hardware
consists of a PLL that converts a RF signal into IF signals.

Depending on the hardware, those video IF signal can be
decoded directly by the bridge chipset. Most Conexant
chips (bt8x8, cx2388x, etc) have internally the decoders
for that. Yet, even on such hardware, the tuner may have
internally its own TV multi-standard decoder like tda9887.

The same happens with the audio IF signal, where some bridges
are capable of receiving it, while others require an external
IF-PLL sound decoder, like msp3400.

Those external IF-PLL audio and video decoders have their own
I2C address, and use different drivers to handle them. So, they're
mapped as different subdevices on Linux.

Thankfully, all modern hardware comes with an IC chip that
has both the RF and the IF stages on it, being capable of
decoding audio and video IF signals internally.

Yet, as we need to support drivers that can work with either
analog or silicon tuners, we need to add two entity types
for those old hardware.

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