drivers: memc: memc_nxp_flexram.h: Move to the public includes directory

Moved to: include/zephyr/drivers/misc/flexram/memc_nxp_flexram.h

This change makes it so that the .h file does not need to be pulled in
using the CMakeLists.txt file, and can be included like other public
includes.

Removes drivers/memc/memc_nxp_flexram.h

Add memc_nxp_flexram.h to include/zephyr/drivers/misc/flexram

Modify drivers/memc/memc_nxp_flexram.c to use the new include path.

Modifies the mimxrt1170 magic_addr sample to include the driver using
the new include path.

Modify the soc file: soc/nxp/imxrt/imxrt11xx/soc.c to use the new path.

Add relevant information to migration-guide-4.2.rst.

Signed-off-by: Jacob Wienecke <jacob.wienecke@nxp.com>
Co-authored-by: Declan Snyder <declan.snyder@nxp.com>