samsung: board: fix: Define loop iterator as an unsigned int to suppress gcc 4.8 warning

This patch suppress following warning:

board/samsung/common/board.c:95:32: warning: iteration 4u invokes undefined behavior [-Waggressive-loop-optimizations]
   addr = CONFIG_SYS_SDRAM_BASE + (i * SDRAM_BANK_SIZE);
                                ^
board/samsung/common/board.c:94:2: note: containing loop

about possible signed integer overflow at gcc 4.8.2 (odroid board)

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>