powerpc/kexec: make the update_cpus_node() function public

Move the update_cpus_node() from kexec/{file_load_64.c => core_64.c}
to allow other kexec components to use it.

Later in the series, this function is used for in-kernel updates
to the kdump image during CPU/memory hotplug or online/offline events for
both kexec_load and kexec_file_load syscalls.

No functional changes are intended.

Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Acked-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240326055413.186534-5-sourabhjain@linux.ibm.com