soc: nxp: imxrt: move FLASH_MCUX_FLEXSPI_XIP_MEM_TARGET to SOC level

FLASH_MCUX_FLEXSPI_XIP_MEM_TARGET definition is required to relocate SOC
level files, as well as Flash and MEMC drivers. Therefore, move the
Kconfig definition to the SOC level, and update the dependencies to
better reflect when the definition is needed.

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