drm: dp: add the definitions for DP Link Compliance tests

Add the definitions for various DPCD register offsets and
corresponding parameter values for DP Link compliance tests
for PHY/audio/link training/link status tests.

Change-Id: If0dd5f7c95b8577aebeada3b225af7f7fde5210d
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>