arm:exynos:gpio: fix s5p_gpio_part_max for exynos4x12

This patch fix wrong value returned by 's5p_gpio_part_max' function
for Exynos4412.

Signed-off-by: Piotr Wilczek <p.wilczek@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>