media: doc-rst: Add Intel IPU3 documentation

This patch adds the details about the IPU3 Imaging Unit driver (both CIO2
and IMGU).
Signed-off-by: Rajmohan Mani <rajmohan.mani@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/media/v4l-drivers/index.rst

@@ -44,6 +44,7 @@ For more details see the file COPYING in the source distribution of Linux.
 	davinci-vpbe
 	fimc
 	imx
+	ipu3
 	ivtv
 	max2175
 	meye