scsi: ufs: dt-bindings: add document for hisi-ufs

add ufs node document for Hisilicon.

Signed-off-by: Li Wei <liwei213@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>