mtd: spi-nor: Compile files in controllers/ directory

Commit a0900d01 ("mtd: spi-nor: Prepare core / manufacturer code
split") moved various files into a new directory, but did not add the new
directory to its parent directory Makefile. The moved files no longer
build, and affected flash chips no longer instantiate.

Adding the new directory to the parent directory Makefile fixes the
problem.

Fixes: a0900d01 ("mtd: spi-nor: Prepare core / manufacturer code split")
Cc: Boris Brezillon <bbrezillon@kernel.org>
Cc: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/spi-nor/Makefile

@@ -18,3 +18,5 @@ spi-nor-objs			+= winbond.o
 spi-nor-objs			+= xilinx.o
 spi-nor-objs			+= xmc.o
 obj-$(CONFIG_MTD_SPI_NOR)	+= spi-nor.o
+
+obj-$(CONFIG_MTD_SPI_NOR)	+= controllers/