dt-bindings: mmc: convert amlogic,meson-gx.txt to dt-schema

Convert the Amlogic SD / eMMC controller for S905/GXBB family SoCs
to dt-schema.

Take in account the used variant with amlogic,meson-gx-mmc.

Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20221117-b4-amlogic-bindings-convert-v3-6-e28dd31e3bed@linaro.org


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