mips/kvm: Implement PRid CP0 register

Implement saving and restoring to KVM state of the Processor ID (PRid)
CP0 register. This allows QEMU to control the PRid exposed to the guest
instead of using the default set by KVM.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
