drm/i915/tgl: Add dkl phy registers

These are the registers needed to program Dekel phy. Some register
definitions will be reused from MG PHY definitions, so adding a
comment on those.

Bspec: 49295

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Clinton A Taylor <clinton.a.taylor@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190920205810.211048-5-jose.souza@intel.com