[media] radio-miropcm20: add RDS support

Once upon a time the radio-miropcm20 driver had RDS support. However, after
some internal kernel changes that support was removed. Now that we have a
nice RDS API I have been working on adding back this support. It has been
tested with the si4713 RDS transmitter and it is working quite nicely.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>