ARM: dts: rockchip: disable gmac by default in rk3288.dtsi

This block should not be enabled by default or else if the kconfig is set,
it will try to load/probe even if there's no phy connected.

Signed-off-by: Alexandru M Stan <amstan@chromium.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>