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 <[e-mail]>
Signed-off-by: Brian Norris <[e-mail]>
Signed-off-by: Felipe Balbi <[e-mail]>

Documentation/devicetree/bindings/usb/dwc3.txt

···
    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
  - phys: from the *Generic PHY* bindings
- - phy-names: from the *Generic PHY* bindings
+ - phy-names: from the *Generic PHY* bindings; supported names are "usb2-phy"
+	or "usb3-phy".
  - snps,usb3_lpm_capable: determines if platform is USB3 LPM capable
  - snps,disable_scramble_quirk: true when SW should disable data scrambling.
 	Only really useful for FPGA builds.