m7: post_boot: Set minimum cpufreq frequency

Also set permissions on cpu0's min/max_freq.

A couple of users have commented that sometimes the minimum
frequency is set to the maximum frequency after boot so force
the minimum frequency.

Taken from http://review.cyanogenmod.org/#/c/56036/

Change-Id: Ia2a221348c8a6980090dfbd5e4f859b933da277c