mtd: spi-nor: allow a flash to define its own ready() function

Xilinx and Micron flashes have their own implementation of the
spi_nor_ready() function. At the moment, the core will figure out
which one to call according to some flags. Lay the foundation to
make it possible that a flash can register its own 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/20220223134358.1914798-19-michael@walle.cc