Add support for 16-bit NAND bus-width for the AT91 NAND driver.

The 16-bit NAND is found on the Atmel AT91SAM9260-EK and AT91SAM9261-EK
boards.

Orignal Patch from Patrice Vilchez
Signed-off-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>