[CPUFREQ] make internal cpufreq_add_dev_* static

No need to export these symbols; make them static.

	cpufreq_add_dev_policy
	cpufreq_add_dev_symlink
	cpufreq_add_dev_interface

Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Dave Jones <davej@redhat.com>