usb: dwc3: gadget: Keep track of IRQ timing statistics

It may be useful to debug how much time is spent in the
DWC3 hard IRQ context. Add low-overhead statistics counters
which keep a histogram of the last 10 interrupts' time of
occurence, time spent, and number of events read (which will
get processed in the bottom half handler).

Change-Id: I466144d8141f4236b9fb834dded93225b56d0422
[jackp@codeaurora.org: Also added BH stats from "usb: dwc3: Offload
 IRQ handling to softirq context"]
Signed-off-by: Jack Pham <jackp@codeaurora.org>