media: imx: Rename functions that add IPU-internal subdevs

For the functions that add and remove the internal IPU subdevice
descriptors, rename them to make clear they are the subdevs internal
to the IPU. Also rename the platform data structure for the internal
IPU subdevices. No functional changes.

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>