drm/dp: Add defines for latency in sink

Add defines for dpcd register 2009 (synchronization latency
in sink).

v2:
 - add spec version (Daniel)
 - use register name as is in spec,only drop excess
   from end (jani)
 - add the full register contents (jani)
[Rodrigo fixed spec version when merging]

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
CC: Puthikorn Voravootivat <puthik@chromium.org>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Vathsala Nagaraju <vathsala.nagaraju@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1506419953-32605-1-git-send-email-vathsala.nagaraju@intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>