ColdFire: Rename NANDFLASH_SIZE into CONFIG_NANDFLASH_SIZE for MCF537x

This patch fixes the build error for MCF537x. As the NANDFLASH_SIZE is
redefined in boards.cfg, it is needed to rename NANDFLASH_SIZE into
CONFIG_NANDFLASH_SIZE in include/configs/M5373EVB.h.

Signed-off-by: Alison Wang <b18965@freescale.com>