mtd: m25p80: add support for Micron N25Q256A

The manufacturer datasheet can be found on the Micron website,
under the name n25q_256mb_3v_65nm.pdf:

http://www.micron.com/search?source=ps&q=n25q_256mb_3v_65nm



Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>