mtd: spi-nor: Add support for mx25u3235f

The mx25u3235f is found on the ZyXEL NBG6817 router, therefore
add driver support for it so that we can upstream board support.

Minimal tested with u-boot tools fw_printenv/fw_setenv on GlobalScale
ESPRESSObin v5 board.

Signed-off-by: André Valentin <avalentin@marcant.net>
[miyatsu@qq.com: Remove unnecessary white space.]
Signed-off-by: Ding Tao <miyatsu@qq.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>