[media] s5p-fimc: Add control of the external sensor clock

Manage the camera sensor clock in the host driver rather than
leaving this task for sensor drivers. The clock frequency
must be passed in the sensor's and host driver's platform data.

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