media: rcar-csi2: Move driver to renesas directory

The rcar-csi2 driver was added before the platform/renesas directory
existed and since it was used together in a pipeline with the rcar-vin
driver it was located together with it.

The rcar-isp driver can also be used together with the rcar-csi2 driver
in a pipeline that is terminated by the rcar-vin driver. However by the
time rcar-isp was added the platform/renesas directory existed so it was
added there.

To remove the confusion that the rcar-csi2 driver have code dependencies
on the rcar-vin driver move it to the same directory level as the
rcar-isp driver. This makes it clear they are three distinct drivers
that can be used together in a pipeline, but do not depend on each
other.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>