drm/i915/ehl: Add voltage level requirement table

EHL has it own voltage level requirement depending on cd clock.

BSpec: 21809
Cc: Clint Taylor <Clinton.A.Taylor@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190626014053.30541-3-jose.souza@intel.com