powerpc/64s: idle POWER9 can execute stop in virtual mode

The hardware can execute stop in any context, and KVM does not
require real mode because siblings do not share MMU state. This
saves a switch to real-mode when going idle.

Acked-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>