Add the required DT parts to enable Ethernet (dwmac-sun8i driver) on
the Nano Pi NEO2 board. It uses an external Realtek RTL8211E PHY
connected via RGMII to provide GbE network. Specially unlike other
Allwinner boards, the phy is connected to MDIO address 7, not 1.

This includes the regulator (which is controlled by a GPIO pin) and
the actual Ethernet MAC node, referring the RGMII pins of the device.
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>