scsi: ufs: qcom: dt-bindings: Add SM8550 compatible string

Document the compatible string for the UFS found on SM8550.

Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>