ARM64: dts: meson s905x: accept MAC addr from u-boot environment

ARM64: dts: meson s905x: accept MAC addr from u-boot environment

With the adequate configuration settings, u-boot will loop through the
list of aliases looking for "ethernetX".

By adding an ethernet alias, u-boot can fixup the local-mac-address
property in the kernel's device tree using a value held in its
environment variable ethaddr.

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