drm/amd/display: Add register definitions for Beige Goby

[Why&How]
Adds registers definitions required for Beige Goby initial support.

Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Chris Park <Chris.Park@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>