dt-bindings: usb: ti,keystone-dwc3.yaml: Add USB3.0 PHY property

AM654 SoC requires USB3.0 PHY to be turned on before the USB controller.
For this SoC, the 'phys' property is used to provide the USB3.0
reference.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>