ARM: imx: disable cpu in .cpu_kill hook

It's buggy to disable the cpu that is being hot-unplugged in .cpu_die
hook which runs on the cpu itself.  Instead, it should be done in
.cpu_kill which runs on the thread (another cpu) that asks for shutting
down the cpu.  Move imx_enable_cpu(cpu, false) call into .cpu_kill
hook, and leave the cpu to be hot-unplugged in WFI within .cpu_die,
so that we can get a more stable cpu hot-plug operation.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
diff --git a/arch/arm/mach-imx/common.h b/arch/arm/mach-imx/common.h
index 7191ab4..fa36fb8 100644
--- a/arch/arm/mach-imx/common.h
+++ b/arch/arm/mach-imx/common.h
@@ -142,6 +142,7 @@
 extern void imx6q_clock_map_io(void);
 
 extern void imx_cpu_die(unsigned int cpu);
+extern int imx_cpu_kill(unsigned int cpu);
 
 #ifdef CONFIG_PM
 extern void imx6q_pm_init(void);