dt-bindings: mmc: Add StarFive MMC module

Add documentation to describe StarFive designware mobile storage
host controller driver.

Signed-off-by: William Qiu <william.qiu@starfivetech.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230215113249.47727-2-william.qiu@starfivetech.com


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