ata: ahci_mvebu: add scsi support

Mvebu AHCI is AHCI driver which uses SCSI under the hood.
This patch adjusts AHCI setup to support SCSI by creating
a SCSI device as a child. Since the functions of creating
SCSI device need the kconfig option DM_SCSI, so let
AHCI_MVEBU select DM_SCSI.

Signed-off-by: Ken Ma <make@marvell.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Stefan Roese <sr@denx.de>