mtd: nand: atmel: prepare for nand spl boot support

Prepare for nand spl boot support. It supports nand software ECC and
hardware PMECC.
This patch is take <drivers/mtd/nand/nand_spl_simple.c> as reference.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>