soc: nxp: imxrt: correct flexspi XIP check to avoid reclocking

RT11xx SOC init should check to see if the zephyr flash node is
set to a device on the FLEXSPI bus to determine if the part is running
in XIP mode. This check was incorrect, so the FLEXSPI was being
reclocked in XIP mode to 24 MHz. Fix this check so the FlexSPI is not
downclocked.

Fixes #75702

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