msm: gsi: data path statistics

Add capabilities to GSI driver to collect data path statistics
in order to help debugging throughput issues.
This change also exposes the a debugfs interface to print stats.

CRs-Fixed: 975119
Change-Id: If97512af034bd419cf4f679b1c19419605529fa6
Acked by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>