target-ppc: Fill in OpenFirmware name for PowerPCCPU models

Extend the POWERPC_DEF*() macros to allow setting DeviceClass::fw_name
in addition to DeviceClass::desc. Use it to store the name to be passed
through flat device tree to OpenFirmware device tree.

Set it to expected values for POWER7, POWER7+, POWER8 and POWER5+.

Signed-off-by: Andreas Färber <afaerber@suse.de>
spapr: Use DeviceClass::fw_name for device tree

Instead of relying on cpu_model (-cpu), obtain the device tree node
label per CPU, using DeviceClass::fw_name where available and populating
it in a PAPR-compliant way otherwise.

This resolves having HOST@0 node labels.

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