nand, davinci: add special UBL ecc position

enable the RBL/UBL ECC layout through
CONFIG_NAND_6BYTES_OOB_FREE_10BYTES_ECC define

see for more info:
http://processors.wiki.ti.com/index.php/DM365_Nand_ECC_layout



Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Tom Rini <trini@ti.com>
Cc: Scott Wood <scottwood@freescale.com>