[media] s5p-fimc: Ensure CAMCLK clock can be enabled by FIMC-LITE devices

In configurations where FIMC-LITE is used to capture image signal
from an external sensor only we need to ensure one of FIMC devices
is in active power state and the "fimc" gate clock is enabled.
Otherwise the CAMCLK clock output signal will be masked off
preventing an external sensor's operation.
This affect processing pipelines like:
 - sensor -> FIMC-LITE -> memory
 - sensor -> MIPI-CSIS -> FIMC-LITE -> memory

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>