exynos78x0: pinctrl: set const to structs

to fix following checkpatch warings.
WARNING: struct  should normally be const

Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>
Cc: Dzmitry Sankouski <dsankouski@gmail.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>