[media] s5p-fimc: Allow defining number of sensors at runtime

Add num_clients field to struct s5p_fimc_isp_info to define exactly
size of clients array which simplifies a bit the sensors management.

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>