media: MAINTAINERS: ipu3-imgu: Add Tian Shu and Bingbu as reviewers

Add Bingbu Cao and Tian Shu Qiu as reviewers for the IPU3 ImgU driver.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: Bingbu Cao <bingbu.cao@intel.com>
Cc: Tian Shu Qiu <tian.shu.qiu@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>