UPSTREAM: usb: dwc3: implement runtime PM

UPSTREAM: usb: dwc3: implement runtime PM

this patch implements the most basic pm_runtime
support for dwc3. Whenever USB cable is dettached,
then we will allow core to runtime_suspend.

Runtime suspending will involve completely tearing
down event buffers and require a full soft-reset of
the IP.

Note that a further optimization could be
implemented once we decide to support hibernation,
which is to allow runtime_suspend with cable
connected when bus is in U3. That's subject to a
separate patch, however.

Change-Id: I2ff3ca0424151eb15066ccc3a6d8e46e7dc9ec4e
Tested-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Wu Liang feng <wulf@rock-chips.com>
(cherry picked from commit fc8bb91bc83ef82868533e75f5a11abc1158ec81)