arm: Add support of updating dts before fix-up

"ethernet" node fix-up for device tree happens before Linux boot.

There can be requirement of updating "ethernet" node even before
fix-up. So, add support of updating "ethernet" node.

Signed-off-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>