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>
Change-Id: I5c5997c6eea817e49102b8e080f3a9bcf45305b4
Git-commit: cf40b86b6ef6df5262ef5a8463b42524e6aa5590
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[hemantk@codeaurora.org: removed write to DWC3_DEV_IMOD in dwc3_interrupt,
and fixed merge conflicts]
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Mayank Rana <mrana@codeaurora.org>