arm64: dts: rockchip: add gmac0 node to rk3568

While both RK3566 and RK3568 feature the gmac1 node, the gmac0
node is exclusive to the RK3568.

Signed-off-by: Michael Riesch <michael.riesch@wolfvision.net>
Link: https://lore.kernel.org/r/20210729093913.8917-2-michael.riesch@wolfvision.net


Signed-off-by: Heiko Stuebner <heiko@sntech.de>