mxrt685: Disabled support for 1.8V USDHC negotiation on RT685 EVK

The IMXRT685 evaluation board uses a PMIC to reduce the signalling
voltage for a connected SD card to 1.8V, which Zephyr does not currently
support. This results in Zephyr failing to interface with any SD card
that supports 1.8V signalling when run on the RT685 eval board. This
commit disables support for 1.8V signalling for the RT685.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>