[POWERPC] Move rtas_stop_self() into platforms/pseries/hotplug-cpu.c

As the first step in consolidating the pseries hotplug cpu code,
create platforms/pseries/hotplug-cpu.c and move rtas_stop_self()
into it.  Do the rtas token initialisation in a new initcall, rather
than rtas_initialize().

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>