net: pxa168_eth: rework the MAC address setup

This patch rework the way the MAC address is retrieved. The MAC address
can now, in addition to being random, be set in the device tree or
retrieved from the Ethernet controller MAC address registers. The
probing function will try to get a MAC address in the following order:
- From the device tree.
- From the Ethernet controller MAC address registers.
- Generate a random one.

This patch also adds a function to read the MAC address from the
Ethernet Controller registers.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>