mtd: spi-nor: Add the SPI_NOR_XSR_RDY flag

S3AN flashes use a specific opcode to read the status register.
We currently use the SPI_S3AN flag to decide whether this specific
SR read opcode should be used, but SPI_S3AN is about to disappear, so
let's add a new flag.

Note that we use the same bit as SPI_S3AN implies SPI_NOR_XSR_RDY and
vice versa.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>