scsi: ceva: Convert driver to use UCLASS_AHCI instead of SCSI

In v2018 the patch
"dm: ahci: Correct uclass private data"
(sha1: bfc1c6b4)
was causing an issue for ceva_sata.
But this issue is not in v2018.05-rc1 but still converting to
UCLASS_AHCI would make more sense.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>