V4L/DVB (13177): radio: Add support for TEF6862 tuner

This patch adds support for TEF6862 Car Radio Enhanced Selectivity Tuner.

It's implemented as a subdev, supporting checking signal strength
and setting and getting frequency.

Signed-off-by: Richard Röjfors <richard.rojfors@mocean-labs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>