dts: arm: nxp_rt5xx_common: disable flexspi by default

Disable flexspi flash controller by default, allow boards to enable it.
This SOC uses external flash, so boards should only enable the flash
controller when their specific flash module has been verified to work
with the flash driver API in Zephyr.

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