usb: gadget: Add snapshot of USB DIAG function driver

This change adds USB DIAG function driver to support diagnostics
functionality with USB transport. This snapshot is taken as of
msm-4.14 commit <9ae54a8e> ("Merge "scsi: ufs-qcom: fix ref-clk
pad regulator initialization"").

Change-Id: Icb70c2d8034add4ae0ac7cc46c3f1778bc10439b
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>