powerpc/B4860QDS: Define new nand_ecclayout structure macros

Define CONFIG_SYS_NAND_MAX_ECCPOS and CONFIG_SYS_NAND_MAX_OOBFREE to
reduce the image size, by taking advantage of the new nand_ecclayout
structure.

Signed-off-by: York Sun <yorksun@freescale.com>
CC: Prabhakar Kushwaha <prabhakar@freescale.com>
CC: Scott Wood <scottwood@freescale.com>