Add DPCD register definitions for eDP 1.4 Multi-SST Operation.

Cc: Nischal Varide <nischal.varide@intel.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ab57627f373ec4a80494bb51ea51080810d9bfb0.1613054234.git.jani.nikula@intel.com