[media] v4l: add control for RF tuner PLL lock flag

Add volatile boolean control to indicate if tuner frequency synthesizer
is locked to requested frequency. That means tuner is able to receive
given frequency. Control is named as "PLL lock", since frequency
synthesizers are based of phase-locked-loop. Maybe more general name
could be wise still?

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>