net: ethernet: enc28j60: add device tree support

The following patch adds the required match table for device tree support
(and while at, fix the indent). It's also possible to specify the
MAC address in the DT blob.

Also add the corresponding binding documentation file.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Signed-off-by: David S. Miller <davem@davemloft.net>