powerpc/64s: POWER9 can execute stop without a sync sequence

We don't need to use IDLE_STATE_ENTER_SEQ_NORET on Power9.

Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Split out of larger patch]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>