net: davinci_emac: use nvmem_get_mac_address()

All DaVinci boards still supported in board files now define nvmem
cells containing the MAC address. We want to stop using the setup
callback from at24 so the MAC address for those users will no longer
be provided over platform data. If we didn't get a valid MAC in pdata,
try nvmem before resorting to a random MAC.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>