dt-bindings: mmc: Add Actions Semi SD/MMC/SDIO controller binding

Add devicetree YAML binding for Actions Semi Owl SoC's SD/MMC/SDIO
controller.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>