usb: xhci: Add support for secondary interrupters

Implement APIs to dynamically allocate and free secondary
event rings based upon interrupter number. Export these
APIs directly from XHCI which allows secondary event ring
management via remote processor entity.

This commit also squashes in the following changes:

    usb: xhci: Clear event handler busy flag upon event ring cleanup
    usb: xhci: Acknowledge pending events in secondary event ring

Change-Id: I5ee7d44d6cad8e35e22d3c1a027a1eec5d208585
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>
Signed-off-by: Jack Pham <jackp@codeaurora.org>