mtd: spi-nor: ids: Add is25wp256 chip

Add is25wp256, chip to spi-nor id table.

Tested on SiFive FU540 board.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>