mtd: spi-nor: Add support for N25Q256A11

Add new Micron N25Q256A (N25Q256A11) 256Mbit NOR Flash in the list
of supported devices. This chip has the same structure as the N25Q256A
but ID and voltage (1V8) to use is different. Therefore, this adds
N25Q256A11 as n25q256ax1.

In the future, for new Micron memories we could use the patterns
"n25q*ax1" for 1V8 and "n25q*ax3" for 3V3 memories.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.kw@hitachi.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>