ARM: socfpga: add smp_ops.cpu_kill to make kexec/kdump available

Kexec_load syscall in ARM requires that machine-specific code
has the smp_ops.cpu_kill() before loading kernel image.
This patch adds the cpu_kill(), as a result, kexec reboot and
kernel crash dump become available in mach-socfpga.

Signed-off-by: Hiraku Toyooka <hiraku.toyooka.gu@hitachi.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Link: http://lists.infradead.org/pipermail/linux-arm-kernel/2015-June/348004.html
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>