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>