[media] MAINTAINERS: Update S5P/Exynos FIMC driver entry

This change is mainly to update the driver's path changed from
drivers/media/platform/s5p-fimc to drivers/media/platform/exynos4-is/.
While at it, remove non-existent files rule, move the whole entry to
the Samsung drivers section and add the patch tracking system site URL.

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>