drm/i915/ehl: Add ElkhartLake platform

Add ElkhartLake as a unique platform as there are some differences
between it and Icelake.

Signed-off-by: Bob Paauwe <bob.j.paauwe@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190322175847.25707-2-rodrigo.vivi@intel.com