Define page retirement functions for MCA platform.

v2: remove page retirement handling from MCA poison handler,
    let MCA notifier do page retirement.

v3: remove specific poison handler for MCA to simplify code.
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>