spapr: move spapr_core_[foo]plug() callbacks close to machine code in spapr.c

spapr: move spapr_core_[foo]plug() callbacks close to machine code in spapr.c

spapr_core_pre_plug/spapr_core_plug/spapr_core_unplug() are managing
wiring CPU core into spapr machine state and not internal CPU core state.
So move them from spapr_cpu_core.c to spapr.c where other similar
(spapr_memory_[foo]plug()) callbacks are located, which also matches
x86 target practice.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>