[media] si2157: implement signal strength stats

Implement DVBv5 signal strength stats. Returns dBm.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Tested-by: Adam Baker <linux@baker-net.org.uk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>