configs: ringneck-px30: fix Ethernet

configs: ringneck-px30: fix Ethernet

Due to updates to the Device Tree (migrating from snps,reset-*
properties to a PHY node on the MDIO bus) in previous commits and the
inability of the designware net driver to support the old Device Tree
(without the MDIO bus and PHY nodes) with DM_MDIO and DM_ETH_PHY
enabled, support for Ethernet on Ringneck PX30 is currently broken.

Now that the transition was made, we can simply enable DM_MDIO and
DM_ETH_PHY to fix Ethernet support on that board. Ideally, we could
squash this commit with the one migrating the Device Tree, but we want
to keep having upstream changes to the Device Trees made by
tools/update-subtree.sh tool without manual intervention.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>