soc: qcom: bam_dmux: Add MTU negotiation feature

Add support for a new feature that allows bam_dmux to negotiate uplink
and downlink MTUs.  This would allow larger data payloads to be sent
across the link thus allowing for better throughput performance through
aggregation.

CRs-Fixed: 641280
Change-Id: I8f0834210049c2cd98146c021284c15466a72db4
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>