boards: ls1046ardb: disable unavailable "ethernet" node in dts

Linux device tree contains "ethernet" node for all possible
interface supported by SoC i.e. LS1046A.

It is not necessary for a SerDes protocol to support all possible
interface. So disable unavailable "ethernet" node in device tree.

Also, enable FDT_SEQ_MACADDR_FROM_ENV to fetch MAC address
sequentially from environment variables

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