arm64: dts: imx8mq-reform2: add uSDHC2 CD pinctrl

The SD card slot on the Reform 2 uses the card detect pad routed to
the uSDHC2 module as intended. This is currently working as it is the
default mux setting for this pad, but better be explicit and add it
to the pinctrl node.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>