arm64: tegra: Add MGBE nodes on Tegra234

Add device tree nodes for the four instances of the Multi-Gigabit
Ethernet (MGBE) IP found on NVIDIA Tegra234 SoCs.

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