sgxfreq: add sysfs interface to store frequency limit

Change-Id: I2d9d1f05b0306de074ffdd327e9bf2a23c9d973f
sgxfreq: limit frequency to default on boot

Change-Id: Ibd6962a71fab7790f3dadfe2a4b31b849b9c3424
libpower: Make it friendly to overclocked CPU

Change-Id: Ie3743fdd5256b3c57a746205fcc7184f86e348ad