mmc: mmci: Add bindings to operate CMD, CK, CKIN pins as GPIO

Add DT bindings to describe GPIO line associated with CMD, CK, CKIN pins.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Ludovic Barre <ludovic.barre@st.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210124170258.32862-1-marex@denx.de


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