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>

MAINTAINERS

@@ -1928,7 +1928,7 @@ F:	Documentation/devicetree/bindings/i2c/i2c-lpc2k.txt
 F:	arch/arm/boot/dts/lpc43*
 F:	drivers/i2c/busses/i2c-lpc2k.c
 F:	drivers/memory/pl172.c
-F:	drivers/mtd/spi-nor/nxp-spifi.c
+F:	drivers/mtd/spi-nor/controllers/nxp-spifi.c
 F:	drivers/rtc/rtc-lpc24xx.c
 N:	lpc18xx