[media] atmel-isi: prepare for the support of preview path

Atmel ISI support a preview path which can output RGB data.

So this patch introduces a bool variable to choose which path is
enabled currently. And also we need setup corresponding path registers.

By default the preview path is disabled. We only use Codec path.

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