[MTD] Driver for AT26Fxxx dataflash devices

Add support for AT26Fxxx dataflash devices. These devices have a quite different
commandset than the AT45xxx chips, which are handled by at91_dataflash.c, so a
combined driver turned out to be more ugly than useful.

Tested only on AT26F004.

Signed-off-by: Hans-Jürgen Koch <hjk@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>