Document PLL lock V4L2 control. It is read only RF tuner control
which is used to inform if tuner is receiving frequency or not.

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