mtd: atmel_nand: add pmecc support for 512, 1k, 4k, 8k page size

PMECC can support 512, 1k, 2k, 4k, 8k page size.
The driver currently only support 2k page size nand flash. So this patch
add support to 512, 1k, 4k and 8k page size nand flash.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>