dmaengine: bcm-sba-raid: Add debugfs support

This patch adds debugfs support to report stats via debugfs
which in-turn will help debug hang or error situations.

Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>