net: macb: Try to retrieve MAC addess from nvmem provider

Call of_get_nvmem_mac_address() to fetch the MAC address from an nvmem
cell, if one is provided in the device tree. This allows the address to
be stored in an I2C EEPROM device for example.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>