SMDK6400: Add some labels to u-boot.lds to support nand_spl

In the nand_spl feature of SMDK6400. Add some relocation symbols to
nand_spl/board/samsung/smdk6400/u-boot.lds to fix the compile error.

Signed-off-by: Zhong Hongbo <bocui107@gmail.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>