usb: dwc3: core: clarify usb-phy array binding

The binding spec wasn't clear that the order of the phandles in the
usb-phy array has meaning.  Clarify this point in the binding that
it should be <USB2-HS-PHY, USB3-SS-PHY>.
Signed-off-by: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>

Documentation/devicetree/bindings/usb/dwc3.txt

@@ -6,7 +6,9 @@ Required properties:
  - compatible: must be "snps,dwc3"
  - reg : Address and length of the register set for the device
  - interrupts: Interrupts used by the dwc3 controller.
- - usb-phy : array of phandle for the PHY device
+ - usb-phy : array of phandle for the PHY device.  The first element
+   in the array is expected to be a handle to the USB2/HS PHY and
+   the second element is expected to be a handle to the USB3/SS PHY
 
 Optional properties:
  - tx-fifo-resize: determines if the FIFO *has* to be reallocated.