platform: msm: Add avtimer device tree documentation

avtimer is used for synchronization of audio and video
during the playback/capture usecase. Add device tree
documentation for avtimer to explain the device tree
node information.

Change-Id: I625a8970e800487746d458c8f51f63c1b371f08c
Signed-off-by: Kuirong Wang <kuirongw@codeaurora.org>