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