dt-bindings: mmc: sdhci-msm: Document compatible for MSM8976

Document the compatible for the SDHCI controller(s) found on MSM8976.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221104172122.252761-6-angelogioacchino.delregno@collabora.com


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