CPUFREQ: PORT FROM R2: expose DOWN_DIFFERENTIAL via sysfs

CPUFREQ: PORT FROM R2: expose DOWN_DIFFERENTIAL via sysfs

BZ: 22582

Expose down_differential through sysfs for experiment with CPU freq
using different workload.

Change-Id: Ic0f4cf55a6a641b42aeed77101d51bb7298c8b73
Signed-off-by: Winson Yung <winson.w.yung@intel.com>
Signed-off-by: Fei Yang <fei.yang@intel.com>
Reviewed-on: http://android.intel.com:8080/34161
Reviewed-by: Gross, Mark <mark.gross@intel.com>
Tested-by: Ng, Cheon-woei <cheon-woei.ng@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>