Add ras poison mode query interface for DF.

v2: replace RREG32_PCIE with RREG32_SOC15.
Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>