powerpc/powernv: Use deepest stop state when cpu is offlined

If hardware supports stop state, use the deepest stop state when
the cpu is offlined.

Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Shreyas B. Prabhu <shreyas@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>