[media] mb86a20s: add CNR measurement

Add Signal/Noise ratio measurement. On this device, a global measure
is taken by the demod. It also provides per-layer CNR measurements,
based on Modulation Error measures (MER).

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