powerpc/64s/idle: Predict HMI wakeup as unlikely

In a busy system, idle wakeups can be expected from IPIs and device
interrupts.

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