KVM: PPC: Book3S HV: Add new POWER9 guest-accessible SPRs

This adds code to handle two new guest-accessible special-purpose
registers on POWER9: TIDR (thread ID register) and PSSCR (processor
stop status and control register).  They are context-switched
between host and guest, and the guest values can be read and set
via the one_reg interface.

The PSSCR contains some fields which are guest-accessible and some
which are only accessible in hypervisor mode.  We only allow the
guest-accessible fields to be read or set by userspace.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

Documentation/virtual/kvm/api.txt

@@ -2023,6 +2023,8 @@ registers, find a list below:
   PPC   | KVM_REG_PPC_WORT              | 64
   PPC	| KVM_REG_PPC_SPRG9             | 64
   PPC	| KVM_REG_PPC_DBSR              | 32
+  PPC   | KVM_REG_PPC_TIDR              | 64
+  PPC   | KVM_REG_PPC_PSSCR             | 64
   PPC   | KVM_REG_PPC_TM_GPR0           | 64
           ...
   PPC   | KVM_REG_PPC_TM_GPR31          | 64