usb: dwc3: add endpoint event rate to debug-fs

For each of the endpoint event counters, add the rate (in Hz)
at which it occurs in order to better analyze performance and
aid in optimization.

Change-Id: I5447a0b0adaa522d529344af67b9fcdb9e5b022f
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>