dt-bindings: mmc: sdhci-sprd: convert to YAML

Covert the Spreadtrum SDHCI controller bindings to DT schema.
Rename the file to match compatible. Drop assigned-* properties as
these should not be needed.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Stanislav Jakubek <stano.jakubek@gmail.com>
Link: https://lore.kernel.org/r/ZozY+tOkzK9yfjbo@standask-GA-A55M-S2HP


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