powerpc/pseries: Call pseries_kexec_setup only on pseries

The pseries_kexec_setup function overwrites some ppc_md
pointers, so make sure it only gets called when running on
the right architecture.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>