dmaengine: Build bcm-sba-raid driver as loadable module for iProc SoCs

By default, we build Broadcom SBA RAID driver as loadable module for
iProc SOCs so that kernel image is little smaller and we load SBA RAID
driver only when required.

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