Register RAS fatal error interrupt and add handler.

v2: only register NBIO RAS for dGPU platform.
    change nbio_v7_9_set_ras_controller_irq_state and nbio_v7_9_set_ras_err_event_athub_irq_state
    to dummy functions.
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>