dt-bindings: mmc: sdhci-msm: Document the SDX55 compatible

The SDHCI controller on SDX55 is based on MSM SDHCI v5 IP. Hence,
document the compatible with "qcom,sdhci-msm-v5" as the fallback.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>