mtd: spi-nor: Prepare core / manufacturer code split

Move all SPI NOR controller drivers to a controllers/ sub-directory
so that we only have SPI NOR related source files under
drivers/mtd/spi-nor/.

Rename spi-nor.c into core.c, we are about to split this file in multiple
source files (one per manufacturer, plus one for the SFDP parsing logic).

Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>