spapr-rtas: add ibm, (get|set)-system-parameter

This adds very basic handlers for ibm,get-system-parameter and
ibm,set-system-parameter RTAS calls.

The only parameter handled at the moment is
"platform-processor-diagnostics-run-mode" which is always disabled and
does not support changing. This is expected to make
"ppc64_cpu --run-mode=1" happy.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
[agraf: s/papameter/parameter/g]
Signed-off-by: Alexander Graf <agraf@suse.de>