dt: bindings: add bindings for Allwinner A64 usb phy

Update sun4i usb phy dt binding documentation to include support for
Allwinner A64 usb phy.
Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>

Documentation/devicetree/bindings/phy/sun4i-usb-phy.txt

@@ -10,6 +10,7 @@ Required properties:
   * allwinner,sun8i-a23-usb-phy
   * allwinner,sun8i-a33-usb-phy
   * allwinner,sun8i-h3-usb-phy
+  * allwinner,sun50i-a64-usb-phy
 - reg : a list of offset + length pairs
 - reg-names :
   * "phy_ctrl"