msm: bam_dmux: add loopback support

Bam dmux is currently very difficult to isolate and provide measurements
for common metrics, such as throughput and latency.

Add hooks into bam_dmux to allow for a seperate dynamically
loadable loopback module. This allows bam dmux to be isolated, and to
provide more accurate measurements of the above metrics.

Change-Id: I174d900dea73cca27d32a54a908f43728059be30
Signed-off-by: Brent Hronik <bhronik@codeaurora.org>