mtd: spi-nor: Add support for S3AN spi-nor devices

Xilinx Spartan-3AN FPGAs contain an In-System Flash where they keep
their configuration data and (optionally) some user data.

The protocol of this flash follows most of the spi-nor standard. With
the following differences:

- Page size might not be a power of two.
- The address calculation (default addressing mode).
- The spi nor commands used.

Protocol is described on Xilinx User Guide UG333

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Marek Vasut <marek.vasut@gmail.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>