media: ipu3.rst: add imgu pipe config tool link

A specific tool which can be used to generate imgu intermedia
resolutions now is public on github, this patch adds this information
into ipu3.rst.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>