drivers: flash_mcux_flexspi_mx25um51345g: move all device data to RAM

since the flexspi driver interacts with the flash device, storing
device data in flash can cause RWW hazards when running in XIP mode.
Move all device data to RAM to limit these RWW hazards.

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