dt-bindings: mmc: convert amlogic,meson-mx-sdio.txt to dtschema

Convert the Amlogic Meson6, Meson8 and Meson8b SDIO/MMC controller
bindings to dt-schema.

Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Message-ID: <20241128-topic-amlogic-arm32-upstream-bindings-fixes-convert-meson-mx-sdio-v4-5-11d9f9200a59@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>