[ARM] 5180/1: at91: Fix at91_nand -> atmel_nand rename fallout

struct at91_nand has been renamed atmel_nand. Fix the four boards that
were added since the patch was created.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Acked-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>