mmc: mmci: Add DT bindings for signal direction

Some variants have support for indicating the bus signal directions,
which currently are configured through platform data.

Add corresponding DT bindings to enable us to move away from using the
platform data.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>