ARM: dts: qcom: Add SoC-specific string for sdhci-msm-v4 nodes

As per upstream discussion [1], we should have an SoC-specific
compatible string for Qualcomm's SDHCI nodes.  Let's add it.

[1] https://lkml.kernel.org/r/20181105203657.GA32282@bogus



Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>