SMDK6400: fix the compiler error

This patch adds _end for fix following compiler error

arch/arm/cpu/arm1176/start.o: In function `_end_ofs':
arch/arm/cpu/arm1176/start.S:61: undefined reference to `_end'
Signed-off-by: NMinkyu Kang <mk7.kang@samsung.com>

board/samsung/smdk6400/u-boot-nand.lds

@@ -67,6 +67,8 @@ SECTIONS
 		*(.dynsym)
 	}
 
+	_end = .;
+
 	.bss __rel_dyn_start (OVERLAY) : {
 		__bss_start = .;
 		*(.bss)