Documentation: dt-bindings: Add snps,need-phy-for-wake for dwc2 USB

Some SoCs with a dwc2 USB controller may need to keep the PHY on to
support remote wakeup.  Allow specifying this as a device tree
property.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

Documentation/devicetree/bindings/usb/dwc2.txt

@@ -42,6 +42,8 @@ Refer to phy/phy-bindings.txt for generic phy consumer properties
 - g-rx-fifo-size: size of rx fifo size in gadget mode.
 - g-np-tx-fifo-size: size of non-periodic tx fifo size in gadget mode.
 - g-tx-fifo-size: size of periodic tx fifo per endpoint (except ep0) in gadget mode.
+- snps,need-phy-for-wake: If present indicates that the phy needs to be left
+                          on for remote wakeup during suspend.
 - snps,reset-phy-on-wake: If present indicates that we need to reset the PHY when
                           we detect a wakeup.  This is due to a hardware errata.
 
@@ -58,4 +60,5 @@ Example:
 		clock-names = "otg";
 		phys = <&usbphy>;
 		phy-names = "usb2-phy";
+		snps,need-phy-for-wake;
         };