soc: nxp_rt6xx: disable flash driver by default

set nxp,imx-flexspi to disabled by default to disable flash driver for
this SOC unless enabled by the target board. This SOC uses external flash,
so boards should enable the flash controller only when flash support
has been verified with their specific external flash module

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