net: ethernet: sun4i-emac: Support phy-handle property for finding PHYs

The sun4i-emac uses the "phy" property to find the PHY it's supposed to
use. This property was deprecated in favor of "phy-handle" in commit
8c5b0944

 ("dt-bindings: net: sun4i-emac: Convert the binding to a
schemas").

Add support for this new property name, and fall back to the old one in
case the device tree hasn't been updated.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: David S. Miller <davem@davemloft.net>