arm64: dts: rockchip: add the gmac power domain on rk3399

This patch adds the gmac ppower-domain to save power consumption
by letting the driver core handle the power-domain so we can
save power on boards not needing Ethernet.

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>