si4708: add RDS support

This adds two IOCTLs.

* SET_RDS enables/disables RDS reading.
* GET_RDS checks if RDS data is available. If so, 4 words of raw RDS
  data are returned. If not, it returns with EAGAIN. This IOCTL must
  be polled approximately every 40ms to get all RDS packets.

Note that RDS is only supported on the SI4709!

Change-Id: I1e98212ecbf637923cb70b55bf30a02ba0b35956
si4708: add mute support

Change-Id: Icbeb65bb03668f45d1eb632d4e1ded8cc2e204a9