mtd: spi-nor: Move Xilinx bits out of core.c

Create a SPI NOR manufacturer driver for Xilinx chips, and move the
Xilinx definitions outside of core.c.

While at it, remove the SPI_S3AN flag which is now useless.

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