common: Fix-up MAC addr in dts by fetching env variable serially

The MAC addresses get fixed in the device tree for "ethernet" nodes
is by using trailing number behind "ethernet" found in "/aliases".
It may not be necessary for the "ethernet" nodes to be sequential.
There can be gaps in between or any node disabled

So provide a support to fetch MAC addr sequentially from env
and apply them to "ethernet" nodes in the order they appear in
device tree only if "ethernet" is not "disabled"

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