[media] v4l2-ctrls: add RX RDS controls

The radio-miropcm20 driver has firmware that decodes the RDS signals. So in that
case the RDS data becomes available in the form of controls.

Add support for these controls to the control framework, allowing the miro driver
to use them.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>