This property has always been supported by the Linux driver; see
commit 9f93ac8d ("net-next: stmmac: Add dwmac-sun8i"). In fact, the
original driver submission includes the phy-supply code but no mention
of it in the binding, so the omission appears to be accidental. In
addition, the property is documented in the binding for the previous
hardware generation, allwinner,sun7i-a20-gmac.

Document phy-supply in the binding to fix devicetree validation for the
25+ boards that already use this property.

Fixes: 0441bde0 ("dt-bindings: net-next: Add DT bindings documentation for Allwinner dwmac-sun8i")
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: David S. Miller <davem@davemloft.net>