MAINTAINERS: update entry after SPI NOR controller move

Commit a0900d01 ("mtd: spi-nor: Prepare core / manufacturer code
split") moved all SPI NOR controller drivers to a controllers/
sub-directory. However, the moved nxp-spifi.c file was referenced in the
ARM/LPC18XX ARCHITECTURE entry in MAINTAINERS.

Hence, since then, ./scripts/get_maintainer.pl --self-test complains:

  warning: no file matches F: drivers/mtd/spi-nor/nxp-spifi.c

Update the file entry in MAINTAINERS to its new location.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>