drm: Adding edp1.4 specific dpcd macros

Adding dpcd macros related to edp1.4 and link rates

v2: Added DP_SUPPORTED_LINK_RATES macros

Signed-off-by: Sonika Jindal <sonika.jindal@intel.com>
Reviewed-by: Todd Previte <tprevite@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>