target/ppc: Introduce 'PowerPCCPUClass::spapr_logical_pvr'

target/ppc: Introduce 'PowerPCCPUClass::spapr_logical_pvr'

Introduce 'PnvChipClass::spapr_logical_pvr' to know corresponding logical
PVR of a PowerPC CPU.
This helps to have a one-to-one mapping between PVR and logical PVR for
a CPU, and used in a later commit to handle cases where PCR of two
generations of Power chip is same, which causes regressions with
compat-mode.

Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Aditya Gupta <adityag@linux.ibm.com>
Tested-by: Amit Machhiwal <amachhiw@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>