media: imx: imx-mipi-csis: Drop unneeded system PM implementation

There's no need to implement system suspend/resume manually, as video
pipelines are supposed to be suspended in a controlled and ordered
manner by the data sink driver at system suspend time (and similarly at
resume time). Drop the system suspend/resume handlers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>