[ARM] Update Acorn ethernet expansion cards

[ARM] Update Acorn ethernet expansion cards
This cset implements validity checks on the ethernet MAC address when
the device is opened, and refuses to open the device if this check
fails.  We also provide the set_mac_address method to allow ifconfig
to change the mac address to something valid.

In addition, the driver is converted from the old expansion card
discovery methods to the new device model driver framework.