arm64: dts: rockchip: enable the gmac for rk3399 evb board

We add the required and optional properties for evb board.
See the [0] to get the detail information.

[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>