mrf24j40: add random extended addr generation

The mrf24j40 has no source to get a permanent extended address. This
patch will add a random generated permanent extended address source.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>