dmaengine: bcm-sba-raid: fix Kconfig dependencies

The new driver requires both mailbox and raid support for compile
testing:

drivers/dma/built-in.o: In function `sba_remove':
edma.c:(.text+0x4414): undefined reference to `mbox_free_channel'
drivers/dma/built-in.o: In function `sba_issue_pending':
edma.c:(.text+0x46cc): undefined reference to `mbox_send_message'
drivers/dma/built-in.o: In function `sba_probe':
edma.c:(.text+0x4e60): undefined reference to `mbox_request_channel'
edma.c:(.text+0x5038): undefined reference to `mbox_free_channel'
drivers/dma/built-in.o: In function `sba_tx_status':
edma.c:(.text+0x5210): undefined reference to `mbox_client_peek_data'

drivers/dma/built-in.o: In function `sba_prep_dma_pq_req':
edma.c:(.text+0x5784): undefined reference to `raid6_gflog'
edma.c:(.text+0x5798): undefined reference to `raid6_gflog'

This rearranges the Kconfig dependencies accordingly.

Fixes: 743e1c8f ("dmaengine: Add Broadcom SBA RAID driver")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>