sun4i_wemac: Use random parts of SID for MAC (if none is specified)

So that the emac gets a unique *and* stable MAC address.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
sun4i_wemac: Add wemac_set_mac_addr() helper function

Signed-off-by: Hans de Goede <hdegoede@redhat.com>