spapr: Move spapr_cpu_init() to spapr_cpu_core.c

spapr: Move spapr_cpu_init() to spapr_cpu_core.c

Start consolidating CPU init related routines in spapr_cpu_core.c. As
part of this, move spapr_cpu_init() and its dependencies from spapr.c
to spapr_cpu_core.c

No functionality change in this patch.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
[dwg: Rename TIMEBASE_FREQ to SPAPR_TIMEBASE_FREQ, since it's now in a
 public(ish) header]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>