hisi_sas: Restrict SCSI_HISI_SAS to arm64

The HiSilicon SAS HBA is available in HiSilicon arm64 SoCs only.
Restrict it to arm64, unless compile-testing.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>