sunxi: update SATA driver to always use DM_SCSI

It seems like the Allwinner SATA driver is already quite capable of
using the driver model, so we can force this on all boards and can
remove support for a non-DM_SCSI build.
This removes the warning about boards with SATA ports not being
DM_SCSI compliant.

It also takes the opportunity to move the driver out of the board/sunxi
directory to join its siblings in drivers/ata, and to make it a proper
Kconfig citizen.

The board defconfigs stay untouched.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@openedev.com>
[jagan: select DM_SCSI separately]
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>