dt-bindings: mmc: sdhci-msm: add sdm670 compatible

The Snapdragon 670 supports eMMC with an SDHCI controller. Add the
appropriate compatible to the documentation.

Signed-off-by: Richard Acayan <mailingradian@gmail.com>
Reviewed-by: Bhupesh Sharma <bhupesh.sharma@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220923014322.33620-2-mailingradian@gmail.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>