KVM: PPC: Book3S HV: Add one-reg interface for HASHPKEYR register

The patch adds a one-reg register identifier which can be used to
read and set the virtual HASHPKEYR for the guest during enter/exit
with KVM_REG_PPC_HASHPKEYR. The specific SPR KVM API documentation
too updated.

Signed-off-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/171759285547.1480.12374595786792346073.stgit@linux.ibm.com