mtd: spi-nor: add support for s25fl208k

Signed-off-by: Sean Nyekjaer <sean.nyekjaer@prevas.dk>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>