drivers: ieee802154: cc2520: User can set the MAC address

Allow user to set the four last bytes of a MAC address.
By default the bytes are selected randomly but for testing
purposes we need a way to specify the address in advance.

Change-Id: Ic9820a8026b327d1ee5025ab77fdc77f5778de67
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>