[media] rtl2832_sdr: convert to platform driver

That SDR driver module was abusing DVB frontend SEC (satellite
equipment controller) device and due to that it was also using
legacy DVB binding. Platform bus is pseudo-bus provided by kernel
driver model and it fits cases like that, where any other busses
are not suitable.

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