arm64: tegra: Enable MGBE on Jetson AGX Orin Developer Kit

A Multi-Gigabit Ethernet (MGBE) instance drives the primary Ethernet
port on the Jetson AGX Orin Developer Kit. Enable it.

Signed-off-by: Bhadram Varka <vbhadram@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>