cpufreq: persistent_stats: make enable node writable only by root

The 'enable' node in the sysfs structure is currently writable by all
users. This is not desirable from an access control perspective. Changing
permissions to provide write access to root only.

Change-Id: I92525cc63571607a38c7aada2a6853cf89b5b38f
Signed-off-by: Anurag Singh <anursing@codeaurora.org>