drm/amd/display: Add DIG_CLOCK_PATTERN register

Add this register for future use

Signed-off-by: Nevenko Stupar <Nevenko.Stupar@amd.com>
Reviewed-by: Vitaly Prosyak <vitaly.prosyak@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>