[POWERPC] pmi probe device by device-type

At the moment the pmi device driver is probing for devices with
a given type and a given name. As there may be devices of
the same type but with a different name, probing should be
done also for device type only.

Signed-off-by: Christian Krafft <krafft@de.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>