usb: dwc3: Implement interrupt moderation

Implement interrupt moderation which allows the interrupt rate to be
throttled. To enable this feature the dwc->imod_interval must be set to
1 or greater. This value specifies the minimum inter-interrupt interval,
in 250 ns increments. A value of 0 disables interrupt moderation.

This applies for DWC_usb3 version 3.00a and higher and for DWC_usb31
version 1.20a and higher.

Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>