[media] v4l2-ctrls: add new RDS TX controls

The si4713 supports several RDS features not yet implemented in the driver.

This patch adds the missing RDS functionality to the list of RDS controls.

The ALT_FREQS control is a compound control containing an array of up
to 25 (the maximum according to the RDS standard) frequencies. To support
that the V4L2_CTRL_TYPE_U32 was added.

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