Add a number of DPCD definitions from DP 1.1 and 1.2a.

v2: drop wrong DP version reference, rename DP training set macros
(Sonika).
Reviewed-by: NSonika Jindal <sonika.jindal@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>