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

Move pSeries_mach_cpu_die() into platforms/pseries/hotplug-cpu.c,
this allows rtas_stop_self() to be static so remove the prototype.

Wire up pSeries_mach_cpu_die() in the initcall, rather than statically
in setup.c, the initcall will still run prior to the cpu hotplug code
being callable, so there should be no change in behaviour.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>