powerpc/powernv: Move cpuidle related code from setup.c to new file

This is a cleanup patch; doesn't change any functionality. Moves
all cpuidle related code from setup.c to a new file.

Signed-off-by: Shreyas B. Prabhu <shreyas@linux.vnet.ibm.com>
Reviewed-by: Preeti U Murthy <preeti@linux.vnet.ibm.com>
[mpe: Fix the SMP=n build by including asm/smp.h in idle.c]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>