generic: write back netdev MAC-address to device-tree

generic: write back netdev MAC-address to device-tree

The label-mac logic relies on the mac-address property of a netdev
devices of-node. However, the mac address can also be stored as a
different property or read from e.g. an mtd device.

Create this node when reading a mac-address from OF if it does not
already exist and copy the mac-address used for the device to this
property. This way, the MAC address can be accessed using procfs.

Signed-off-by: David Bauer <mail@david-bauer.net>