[media] v4l: atmel-isi: Remove support for platform data

All in-tree users have migrated to DT, remove support for platform data.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[josh.wu@atmel.com: squash the commit to remove the unused variable: dev]
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>