[POWERPC] Wrap cpu_die() with CONFIG_HOTPLUG_CPU

Per email discussion, it appears that rtas_stop_self()
and pSeries_mach_cpu_die() should not be compiled if
CONFIG_HOTPLUG_CPU is not defined. This patch adds
#ifdefs around these bits of code.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>