[media] exynos4-is: Add Exynos4x12 FIMC-IS device tree binding documentation

This patch adds DT binding documentaton for the Imaging Subsystem
(camera ISP) found on Samsung Exynos4x12 SoCs.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>