powerpc: Remove unnecessary CONFIG_SMP #ifdefs

The code in machine_restart/power_off/halt() includes #ifdefs around
calls to smp_send_stop(), however these are not required as
include/linux/smp.h includes an empty version of this function for
CONFIG_SMP=n builds.

Signed-off-by: Chris Smart <chris@distroguy.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>