media: docs: ipu3.rst: rely at automarkup extension

There are several :c:type: definitions there, in order to
do cross-references with the driver's documentation.

Those are broken when docs are built with Sphinx 3.x, as
it would require :c:struct: instead.

For Sphinx < 3.x, the automarkup.py extension is able to do the
replacement already, and a future improvement on it should make
it also work with Sphinx 3.x.

So, get rid of the usage of the :c:type: macro there.

Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>