spapr: Use DeviceClass::fw_name for device tree CPU node

spapr: Use DeviceClass::fw_name for device tree CPU node

Instead of relying on cpu_model, obtain the device tree node label
per CPU. Use DeviceClass::fw_name as source.

Whenever DeviceClass::fw_name is unknown, default to "PowerPC,UNKNOWN".

As a consequence, spapr_fixup_cpu_dt() can operate on each CPU's fw_name,
obsoleting sPAPREnvironment::cpu_model, and spapr_create_fdt_skel() can
drop its cpu_model argument.

Signed-off-by: Prerna Saxena <prerna@linux.vnet.ibm.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexander Graf <agraf@suse.de>