kconfig: flash: Move mcux code into it's own file

Cleanup the Kconfig code for the flash driver. Platform-specific
options should be in their own Kconfig files to be consistent and to
not pollute the common configuration.

To this end we move the mcux options into it's own file.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
2 files changed