[media] v4l2: Add a V4L2 driver for SI476X MFD

This commit adds a driver that exposes all the radio related
functionality of the Si476x series of chips via the V4L2 subsystem.

[mchehab@redhat.com: change it to depends on MFD_SI476X_CORE instead of
 selecting it; vidioc_s_register now uses const struct]
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>