scsi: ufs: dt-bindings: Add document for Unisoc UFS host controller

Add Unisoc ums9620 UFS host controller devicetree document.

Signed-off-by: Zhe Wang <zhe.wang1@unisoc.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>