powerpc/powernv: abstraction for saving SPRs before entering deep idle states

Create a function for saving SPRs before entering deep idle states.
This function can be reused for POWER9 deep idle states.

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>