generic: add mac-address property for NVMEM mac addresses

generic: add mac-address property for NVMEM mac addresses

Traversing the device-tree by referencing a network device to determine
a devices labe-mac does not work with the generic nvmem implementation,
as the userspace expects the MAC-address to be available as a
device-tree property.

The legacy mtd-mac-address implementation did create such a node. Do the
same when using the nvmem implementation to allow reading the MAC
address.

Fixes commit d284e6ef0f06 ("treewide: convert mtd-mac-address-increment*
to generic implementation")

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