dt-bindings: net: Add Tegra234 MGBE

Add device-tree binding documentation for the Multi-Gigabit Ethernet
(MGBE) controller found on NVIDIA Tegra234 SoCs.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Bhadram Varka <vbhadram@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>