[media] V4L: sh_mobile_csi2: switch away from using the soc-camera bus notifier

This moves us one more step closer to eliminating the soc-camera bus
and devices on it. Besides, as a side effect, CSI-2 runtime PM on
sh-mobile secomes finer grained now: we only have to power on the
interface, when the device nodes are open.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>