arm64: dts: rockchip: add the gmac needed node for rk3399

The RK3399 GMAC Ethernet Controller provides a complete Ethernet interface
from processor to a Reduced Media Independent Interface (RMII) and Reduced
Gigabit Media Independent Interface (RGMII) compliant Ethernet PHY.

This patch adds the related needed device information.
e.g.: interrupts, grf, clocks, pinctrl and so on.

The full details are in [0].

[0]:
Documentation/devicetree/bindings/net/rockchip-dwmac.txt

Signed-off-by: Roger Chen <roger.chen@rock-chips.com>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>