drm/amd/include: add bitfield define for EDC registers

Add EDC registers to support VEGA20 RAS

Signed-off-by: Dennis Li <Dennis.Li@amd.com>
Reviewed-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>