mini2440: add a new macro to describe nand page size

Signed-off-by: Fudong Bai <fudongbai@gmail.com>
mini2440: introduced new config option

CONFIG_LL_INIT_NAND_ONLY to perform low-level initialization only when
booting from NAND

Signed-off-by: Fudong Bai <fudongbai@gmail.com>