drivers: base: cpu: fix compiler warning

Linux Kernel 3.18 introduced few compile-time warnings, fix
them.

1. /kernel/drivers/base/cpu.c:317:12: warning: 'cpu_uevent' defined but not used [-Wunused-function]
 static int cpu_uevent(struct device *dev, struct kobj_uevent_env *evt)

Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>