arm64: dts: rockchip: add gmac to rk3308 dts

The RK3308 SoC has a gmac with only the RMII interface exposed. This
commit adds it to the RK3308 dtsi.

Signed-off-by: Tobias Schramm <t.schramm@manjaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>