dt-bindings: Add DT bindings document for Broadcom SBA RAID driver

This patch adds the DT bindings document for newly added Broadcom
SBA RAID driver.

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