Saving off CPU MHZ.
Saving off CPU MHZ.

The CPU MHZ file is looked at later on by send_metrics_on_resume
script when logging the firmware resume time metric.

TEST=rebooted the system and verified that the MHZ file gets created correctly
BUG=chromium-os:4788

Change-Id: I3b2dca4d1df2d3b8f9e26d030ac38b0e99f98230

Review URL: http://codereview.chromium.org/3539020

joshfire/chromiumos-platform-init