ath79: use hardware MDIO on MikroTik RouterBOARD wAP G-5HacT2HnD

ath79: use hardware MDIO on MikroTik RouterBOARD wAP G-5HacT2HnD

Instead of using a bit-banged GPIO, configure the GPIO output mux to use
the SoC's MDIO functionality. According to the QCA955x datasheet, MDIO1
is associated with the use of SGMII.

The following changes are also made:
* Drop phy-mode from phy0: the ar71xx driver looks for this property in
  ethX nodes, but the correct value is already specified in the dtsi.
* Drop the deprecated 'device-type = "ethernet-phy"' for phy0.

Signed-off-by: Sander Vanheule <[email protected]>
[drop mdio2 from aliases]
Signed-off-by: Adrian Schmutzler <[email protected]>