dt-bindings: ufs: Add msm8998 compatible string

Add "qcom,msm8998-ufshc" compatible string.

Tested-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Marc Gonzalez <marc.w.gonzalez@free.fr>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>