lib: s390x: smp: refactor smp functions to accept indexes

Refactor all the smp_* functions to accept CPU indexes instead of CPU
addresses.

Add SIGP wrappers to use indexes instead of addresses. Raw SIGP calls
using addresses are still possible. Some of the reworked functions
also use the new wrappers internally, for clarity, even though that
will cause unnecessary redundant checks on the validity of the CPU
index.

Add a few other useful functions to deal with CPU indexes.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>