s390/qeth: add support for early L3 device setup

Similar to how qeth currently does early L2 setup of OSM and OSN
devices, add support for early setup of L3-only devices.
This adds a qeth_l3_devtype that contains all core and l3-specific
sysfs attributes, so that they can be created in one go while probing.

This just adds the infrastructure, exploitation of the support happens
in a subsequent patch.

Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Reviewed-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>