This adds radio receiver and transmitter support. Part of that is common
to both and so is placed in the radio-common source.

These drivers also support RDS. In order to generate valid RDS data a
simple RDS generator is implemented in rds-gen.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>