net: w5100-spi: add support to specify MAC address by device tree

This adds support to specify the MAC address by 'mac-address' or
'local-mac-address' properties in the device tree.  These are common
properties for the Ethernet controller.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Mike Sinkovsky <msink@permonline.ru>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>