[POWERPC] Cleanup pseries kexec code

Move all the pseries kexec code into one file, platforms/pseries/kexec.c
Provide helpers for setting up ppc_md.kexec_cpu_down, so that we don't
have to have #ifdef CONFIG_KEXEC in setup.c

Move the initialisation of the ppc_md kexec callbacks into an init routine.
This is well and truly early enough to cause no change in behaviour, we
can't kexec until userspace has given us a kernel to kexec into.

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