media: doc-rst: add yavta test example in ipu3 docs

This patch add yavta test command in ipu3.rst as an example on how to
run simple ImgU test using yavta.

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+huawei@kernel.org>