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>