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: Romain Perier <romain.perier@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>