Documentation: dt: dwc3: note the supported phy-names

The dwc3 driver expicitly looks for "usb2-phy" or "usb3-phy", but we
never noted these names in the documentation.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>