media: ipu3.rst: add explanation for running mode

Add some explanation of the ImgU running mode and add more information
about firmware selection and running mode usage.

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>