mtd: spi-nor: Add support for is25wp256

Update the spi_nor_id table for is25wp256 (32MB) device from ISSI,
present on HiFive Unleashed dev board (Rev: A00).

Use the post bfpt fixup hook for the is25wp256 device, as done for
the is25lp256 device to overwrite the wrong address width advertised
by BFPT.

Signed-off-by: Sagar Shrikant Kadam <sagar.kadam@sifive.com>
[tudor.ambarus@microchip.com: rebase, split and adapt for latest spi-nor/next]
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Reviewed-by: Vignesh Raghavendra <vigneshr@ti.com>