powerpc: Provide dummy crash_shutdown_register

When kexec is disabled, the crash_shutdown_{un,}register
functions are not available in the kernel.
This provides dummy inline functions for those so that
the callers don't have to worry about it.

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