drm/cec: Add CEC over Aux register definitions

Adding DPCD register definitions from the DP 1.3 specification for CEC
over AUX support.

V2: Add DP_ prefix to all defines.
V3: missed prefixes from the ESI1 defines

Cc: Jani Nikula <jani.nikula@linux.intel.com>

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Clint Taylor <clinton.a.taylor@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1492703263-11494-1-git-send-email-clinton.a.taylor@intel.com