usb: dwc3: Offload IRQ handling to softirq context

Move interrupt handling from threaded IRQ context to a tasklet.
This may help in reducing latencies associated with starvation
caused by high priority softirq over threaded USB IRQ.
Also added stats to see number of events processed in tasklet.
These stats are logged in int_events file. It shows the
number of events handled by tasklet and time taken in usec for
its completion for last 10 interrupts.

Change-Id: I2f988be9c484a6ee59236d9dfd7f06f07414ee96
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Azhar Shaikh <azhars@codeaurora.org>