platform: msm: Add avtimer device tree documentation

* avtimer

Avtimer provides an interface for clients to enable avtimer block

on qdsp6. 64 bit AVtimer exposed by qdsp6 is used for audio and video

stream synchronization during capture and playback usecases.

Required properties:

- reg : physical address and length of avtimer register

- reg-names : AVtimer register name

  Required register resource entries are:

  "avtimer_lsb_addr" : AVtimer lsb physical address

  "avtimer_msb_addr" : AVtimer msb physical address

- compatible : Must be "qcom,avtimer"

Optional properties:

- clk-div : The clk is at 27MHz and hence ticks are to be

 divided by 27 to achive the msec value.

Example:

	qcom,avtimer@90f7000 {

		compatible = "qcom,avtimer";

		reg = <0x90f700c 0x4>,

		      <0x90f7010 0x4>;

		reg-names = "avtimer_lsb_addr", "avtimer_msb_addr";

		qcom,clk-div = <27>;

	};