ARM64: dts: meson: accept MAC addr from u-boot environment

Extend configuring the MAC address from u-boot to all meson boards.

I didn't test this changeset but having checked libretech's u-boot
tree I believe it should just work.

Signed-off-by: Jorge Ramirez-Ortiz <jramirez@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>