mtd: mchp23k256: Add driver for this SPI SRAM device

The Microchip 23k256 is a 32K Byte SRAM connected via SPI.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
[Brian: fixed copyright to be in this millenium]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>