[media] atmel-isi: add runtime pm support

The runtime pm resume/suspend will enable/disable pclk (ISI peripheral
clock).

We have to call runtime_pm_get_sync()/runtime_pm_put() when we need to
access ISI registers. In atmel_isi_probe(), remove the isi disable code
as at that moment ISI peripheral clock is not enable yet.

Besides, clock_start()/clock_stop() is used to control the mclk, not
the ISI peripheral clock. So move this to start[stop]_streaming()
function.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>