mmc: mmci: Add DT bindings for feedback clock pin

The ST Micro variant supports the option of using a feedback clock signal in
favor of the clockout pin when latching incoming signals on the data bus.

Since this is matter of how pins are being routed we need to provide a new DT
binding to be able to configure this through DT.

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