drm/i915/skl: Updated the i915_frequency_info debugfs function

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: Akash Goel <akash.goel@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>