exynos: mmu: use CONFIG_IS_ENABLED

to fix following checkpatch warnings.
Use 'if (IS_ENABLED(CONFIG...))' instead of '#if or #ifdef' where
possible

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