[media] cx24120: Add DVBv5 signal strength stats

Add new get_stats function, called from read_status, for
collecting DVBv5 stats into the frontend cache. Only signal
strength for now, can add others later.
Not currently marked as available, future patch will enable.

Signed-off-by: Jemma Denson <jdenson@gmail.com>
Signed-off-by: Patrick Boettcher <patrick.boettcher@posteo.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>