usb: xhci: add xhci_log_ring trace events

This patch creates a new event class called xhci_log_ring, and
defines the events used for tracing the change of all kinds of
rings used by an xhci host. An xHCI ring is basically a memory
block shared between software and hardware. By tracing changes
of rings, it makes the life easier for debugging hardware or
software problems.

This info can be used, later, to print, in a human readable way,
the life cycle of an xHCI ring using the trace-cmd tool and the
appropriate plugin.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>