drm/i915/dg2: Add cdclk table and reference clock

Note that DG2 only has a single possible refclk frequency (38.4 MHz).

v2:
 - Drop two now-unused cdclk entries

Bspec: 54034
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210721223043.834562-12-matthew.d.roper@intel.com