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>