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>

arch/arm/mach-imx/common.h

@@ -142,6 +142,7 @@ extern int imx6q_set_lpm(enum mxc_cpu_pwr_mode mode);
 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);

arch/arm/mach-imx/hotplug.c

@@ -46,9 +46,11 @@ static inline void cpu_enter_lowpower(void)
 void imx_cpu_die(unsigned int cpu)
 {
 	cpu_enter_lowpower();
-	imx_enable_cpu(cpu, false);
+	cpu_do_idle();
+}
 
-	/* spin here until hardware takes it down */
-	while (1)
-		;
+int imx_cpu_kill(unsigned int cpu)
+{
+	imx_enable_cpu(cpu, false);
+	return 1;
 }

arch/arm/mach-imx/platsmp.c

@@ -92,5 +92,6 @@ struct smp_operations  imx_smp_ops __initdata = {
 	.smp_boot_secondary	= imx_boot_secondary,
 #ifdef CONFIG_HOTPLUG_CPU
 	.cpu_die		= imx_cpu_die,
+	.cpu_kill		= imx_cpu_kill,
 #endif
 };