net: mvneta: support setting hardware address

mvneta already supports setting the MAC address but this was only done
internally when some other part of U-Boot tries to actually use the
interface. This commit exposes this functionality to the ethernet core
code so that the MAC addresses of all interfaces are configured
correctly even if they are not used before loading Linux.

Signed-off-by: Matt Pelland <mpelland@starry.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>