[media] s5p-csis: Allow to specify pixel clock's source through platform data

Depending on the sensor configuration it might be required to adjust
the CSIS's output pixel clock so it is greater than its input pixel
clock, in order to avoid the input data FIFO overflow.
Use platform data to select SCLK_CSIS clock from CMU as a source, rather
than CSI APB clock.

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>