drm/i915/skl: Updated the i915_ring_freq_table debugfs function

Updated the i915_ring_freq_table debugfs function to support the read
of ring frequency table, through Punit interface, for SKL also.

Issue: VIZ-5144
Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>