ARC: [plat-axs*/plat-hsdk]: Allow U-Boot to pass MAC-address to the kernel

Otherwise kernel uses random MAC which is not very conveniet.
With that change in place use might set desired MAC in U-Boot
with "setenv ethaddr 11:22:33:44:55:66", save environment and
then from boot to boot the same MAC will be used by the kernel.

One other note for this to happen it's required to pass
board's .dtb in U-Boot's "bootm" command like that:
------------------->8-----------------
bootm 0x82000000 - 0x84000000
------------------->8-----------------

Here 0x82000000 is location of uImage while
0x80000000 is location of either axs10x.dtb or hsdk.dtb
previously loaded from SD-card, USB storage or TFTP server.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: stable@vger.kernel.org # 4.14
Cc: devicetree@vger.kernel.org
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>