Adding dpcd macros related to edp1.4 and link rates

v2: Added DP_SUPPORTED_LINK_RATES macros
Signed-off-by: NSonika Jindal <sonika.jindal@intel.com>
Reviewed-by: NTodd Previte <tprevite@gmail.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>