mtd: OneNAND: add support for OneNAND manufactured by Numonyx

This patch adds the Numonyx manufacturer code (0x20) to
onenand manufacturers.

Signed-off-by: Enric Balletbo i Serra <eballetbo@gmail.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Steve Sakoman <steve.sakoman@linaro.org>
Tested-by: Steve Sakoman <steve.sakoman@linaro.org>