mtd: spi-nor: move all xilinx specifics into xilinx.c

Mechanically move all the xilinx functions to its own module.

Then register the new flash specific ready() function.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Pratyush Yadav <p.yadav@ti.com>
Link: https://lore.kernel.org/r/20220223134358.1914798-22-michael@walle.cc