ARM: tango: fix CONFIG_HOTPLUG_CPU=n build

Building with CONFIG_HOTPLUG_CPU disabled fails for mach-tango:

include/linux/stddef.h:7:14: error: excess elements in struct initializer [-Werror]
 #define NULL ((void *)0)
arch/arm/mach-tango/platsmp.c:48:15: note: in expansion of macro 'tango_cpu_kill'
  .cpu_kill  = tango_cpu_kill,

the problem as that the .cpu_kill and .cpu_die struct members are
unavailable and we must not try to assign them in this configuration.

Hiding the two (as all other platforms do too) lets us get rid of
the #else clause as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Olof Johansson <olof@lixom.net>