arm64: dts: imx8mm-venice-gw7: Adjust PCI Ethernet nodes

As per Rob Herring's feedback:

"The ethernet device should have a node name of
'ethernet'. The 'pcie' node name and 'device_type = "pci"' is for PCI
buses/bridges only."

Do it as suggested.

Fixes: d61c5068729a ("arm64: dts: imx8mm-venice-gw7: Fix pci sub-nodes")
Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>