dt-bindings: Document phy-supply property in rockchip-dwmac

As no property for phy regulator was documented in this dt-binding documentation,
this commit adds an entry for the optional property phy-supply.
Signed-off-by: NRomain Perier <romain.perier@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/rockchip-dwmac.txt

@@ -33,6 +33,7 @@ Required properties:
 Optional properties:
  - tx_delay: Delay value for TXD timing. Range value is 0~0x7F, 0x30 as default.
  - rx_delay: Delay value for RXD timing. Range value is 0~0x7F, 0x10 as default.
+ - phy-supply: phandle to a regulator if the PHY needs one
 
 Example: