powerpc/powernv: pnv_init_idle_states() should only run on powernv

Although this init call checks for device tree properties before doing
anything, it should still only run on powernv machines.

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