dpaa_eth: Use random MAC address when none is given

If there is no valid MAC address in the device tree, use a random
MAC address.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Madalin Bucur <madalin.bucur@oss.nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>