ARM: DTS: stm32: add MMC nodes for stm32f746-disco and stm32f769-disco

Add DT nodes to enable ARM_PL180_MMCI IP support for STM32F746
and STM32F769 discovery boards

There is a hardware issue on these boards, it misses a pullup on the GPIO line
used as card detect to allow correct SD card detection.
As workaround, cd-gpios property is not present in DT.
So SD card is always considered present in the slot.

Signed-off-by: Christophe Priouzeau <christophe.priouzeau@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>