ppc4xx: Move definition for PPC4xx NAND FLASH controller to header

This patch moves the definition for the PPC4xx NAND FLASH controller
(NDFC) CONFIG_NAND_NDFC into include/ppc4xx.h. This is needed for the
upcoming fix for the ECC byte ordering of the NDFC driver.

Signed-off-by: Stefan Roese <sr@denx.de>
Acked-by: Scott Wood <scottwood@freescale.com>