ARM: tegra: Refactor DT update helpers

Rather than duplicate the Ethernet MAC address and carveout updating
code for each board, move it to a common location and make it more
reusable.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>