dts: provide card detect GPIOs connected to uSDHC2

On most i.MX-based systems, including the MNT Reform2, the SD2 card
detect pin is routed to a GPIO instead of the uSDHC itself.  The OS
can only properly do card detect, if it is told that there is a GPIO
to look at.  This is also necessary for the MNT Reform2.

While the device tree currently does not specify any IOMUX config
for this pin, the U-Boot built for this hardware already sets it up.
It would make sense to add this pinctrl to the device tree as well.
2 jobs for bluerise-sd2-cd
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failed build #378

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