[media] v4l: atmel-isi: Make the MCK clock optional

ISI_MCK is the sensor master clock. It should be handled by the sensor
driver directly, as the ISI has no use for that clock. Make the clock
optional here while platforms transition to the correct model.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>