msm: bam_dmux: add power management only mode

Add an internal power management only mode for when the bam_dmux will
not be used as a data multiplexer, but solely for its A2 power collapse
feature support for other clients which may be interacting directly with
the A2.

CRs-Fixed: 372963
Change-Id: I75c8d3f5fa07318caa05c0c3ee5fe029b9874f13
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>