net: arc: rockchip: fix emac mdio node support

The binding emac_rockchip.txt is converted to YAML.
Changed against the original binding is an added MDIO subnode.
This make the driver failed to find the PHY, and given the 'mdio
has invalid PHY address' it is probably looking in the wrong node.
Fix emac_mdio.c so that it can handle both old and new
device trees.

Fixes: 1dabb749 ("ARM: dts: rockchip: restyle emac nodes")
Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Tested-by: Andy Yan <andyshrk@163.com>
Link: https://lore.kernel.org/r/20220603163539.537-3-jbx6244@gmail.com


Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>