Update the mtd driver name in bootargs for at91-based boards

Update the mtd driver name in bootargs for at91-based boards

The name of the atmel nand driver in the kernel changed from at91_nand
to atmel_nand back in June 2008, but the at91-based boards config files
still refer to at91_nand. This patch updates them with the new name

Signed-off-by: Albin Tonnerre <albin.tonnerre@free-electrons.com>