drm/msm/dp: add debugfs support for restricting maximum link rate

Add debugfs support to restrict the maximum link rate supported by
Display Port driver. This is needed for certain cases of DP Link CTS
where the sink supports maximum link rate of 5.4 GHz. Any link rate
which is published more than 5.4 GHz in DPCD is expected by the source
to be treated as error and fallback to fail-safe of 1.62 GHz.

Change-Id: Ief14586b45fb832c2fd00a6372ddf57b3ace5864
Signed-off-by: Padmanabhan Komanduru <pkomandu@codeaurora.org>