ARM: OMAP: Fix section mismatch warning for platform_cpu_die()

WARNING: vmlinux.o(.text+0x226d0):
Section mismatch in reference from the function
platform_cpu_die() to the function .cpuinit.text:omap4_hotplug_cpu()
The function platform_cpu_die() references
the function __cpuinit omap4_hotplug_cpu().
This is often because platform_cpu_die lacks a __cpuinit
annotation or the annotation of omap4_hotplug_cpu is wrong.

Thanks to Russell King for suggesting to use __ref instead of
the initial (and wrong) approach to use __cpuinit.
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/mach-omap2/omap-hotplug.c

@@ -33,7 +33,7 @@ int platform_cpu_kill(unsigned int cpu)
  * platform-specific code to shutdown a CPU
  * Called with IRQs disabled
  */
-void platform_cpu_die(unsigned int cpu)
+void __ref platform_cpu_die(unsigned int cpu)
 {
 	unsigned int this_cpu;