net: mvneta: read MAC address from hardware when available

This patch improves the logic used by the mvneta driver to find a MAC
address for a particular interface. Until now, it was only looking at
the Device Tree, and if no address was found, was falling back to
generating a random MAC address.

This patch adds the intermediate solution of reading the MAC address
from the hardware registers, in case it has been set by the
bootloader. So the order is now:

 1) MAC address from the Device Tree
 2) MAC address from the hardware registers
 3) Random MAC address

This requires moving the MAC address initialization a little bit later
in the ->probe() code, because it now requires the hardware registers
to be remapped.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>