usb: dwc3: core: add a event buffer cache

This extra buffer will be used so we can copy
triggered events from our event buffer to this cache
and process all of them later in bottom half
handler.

We need this in order to implement a workaround for
a known erratum in recent DWC3 release.

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