dt-bindings: mmc: add DT bindings for Milbeaut SD controller

Add the device-tree binding documentation for Milbeaut SDHCI driver.

Signed-off-by: Takao Orito <orito.takao@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>