mmc: mmci: Convert bindings to DT schema

This converts the MMCI bindings from simple text to a proper
schema.

Cc: devicetree@vger.kernel.org
Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20210119100655.2755164-1-linus.walleij@linaro.org


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