ARM: socfpga: hide unused functions

The cpu_die and cpu_kill callbacks are only used when CONFIG_HOTPLUG_CPU
is enabled, otherwise we get a warning about them:

arch/arm/mach-socfpga/platsmp.c:102:13: error: 'socfpga_cpu_die' defined but not used [-Werror=unused-function]
arch/arm/mach-socfpga/platsmp.c:115:12: error: 'socfpga_cpu_kill' defined but not used [-Werror=unused-function]

This adds the appropriate #ifdef.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Dinh Nguyen <dinguyen@opensource.altera.com>