[media] s5p-fimc: Fix FIMC3 pixel limits on Exynos4

Correct pixel limits for the fourth FIMC entity on Exynos4 SoCs.
FIMC3 only supports the writeback input from the LCD mixer.
Also rename s5pv310 variant to exynos4 which is needed after
renaming s5pv310 series to Exynos4.

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>