[media] exynos4-is: Add fimc-is subdevs registration

This patch adds support for registration of the FIMC-IS device
represented by the FIMC-IS-ISP subdev to the top level media device
driver. The FIMC-IS subsystem is available on Exynos4x12 SoCs which
support only device tree based booting.

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