V4L/DVB: V4L2: soc-camera: add a MIPI CSI-2 driver for SH-Mobile platforms

Some SH-Mobile SoCs implement a MIPI CSI-2 controller, that can interface to
several video clients and send data to the CEU or to the Image Signal
Processor.  This patch implements a v4l2-subdevice driver for CSI-2 to be used
within the soc-camera framework, implementing the second subdevice in addition
to the actual video clients.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>