[media] it913x: change driver model from i2c to platform

That tuner is integrated to demodulator and communicates via
demodulators address space. We cannot register both demodulator
and tuner having same address to same I2C bus, so better to change
it platform driver in order to implement I2C adapter correctly.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>