Documentation: Add iris_fm device to device tree

Qti radio iris device

-FM RX playback with no RDS

   FM samples is filtered by external RF chips at baseband, then send to Riva-FM core through serial link.

   FM signal is demodulated then audio L/R samples are stored inside memory.

   FM Rx received samples data is connected to external audio codec.

-Audio playback to FM TX

  Used to play audio source  to FM TX.

  FM TX module will read the audio samples from memory then modulated samples will be send through serial interface to external RF chip.

-RX playback with RDS

  FM Rx receive audio data along with RDS.

-FM TX with RDS

  Used to send RDS messages to external FM receiver.

Required Properties:

- compatible: "qcom,iris_fm"

Example:

	qcom,iris-fm {

		compatible = "qcom,iris_fm";

	};