ARM: tegra: configure Ethernet address on Tegra186

On Tegra186, the bootloader which runs before U-Boot passes the Ethernet
MAC address to U-Boot using device tree. Extract this value and write it
to the environment, so that the Ethernet uclass picks it up and uses it
for the built-in Ethernet device.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>