dt-bindings: mmc: sdhci-msm: document resets

dt-bindings: mmc: sdhci-msm: document resets

Commit "mmc: sdhci-msm: Reset GCC_SDCC_BCR register for SDHC" added
support for utilizing a hardware reset and parsing it from DT, however
the bindings were not updated along with it.

So, document the usage of "resets" property with the limit of only one
item.

Signed-off-by: Robert Marko <robimarko@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220704143554.1180927-1-robimarko@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>