net: eth-uclass: Support device tree MAC addresses

Add the standard Ethernet device tree bindings (imported from v5.0 of
the Linux kernel) and implement support for reading the MAC address for
Ethernet devices in the Ethernet uclass. If the "mac-address" property
exists, the MAC address will be parsed from that. If that property does
not exist, the "local-mac-address" property will be tried as fallback.

MAC addresses from device tree take precedence over the ones stored in
a network interface card's ROM.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>