Added support for SKL in the i915_frequency_info debugfs function

v2:
- corrected the handling of reqf (Damien)
- Reorderd the platform check for cagf (Ville)
Signed-off-by: NAkash Goel <akash.goel@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>