[media] s5p-csis: Add missing dependency on PLAT_S5P

s5p-csis is to be built only on S5P SoC platforms.

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