media: MAINTAINERS: Fix Hantro, Rga and Rkvdec entries

It seems recent merges introduced a couple issues
here, so let's fix them all. Also, reorder Rockchip
video decoder as per parse-maintainers.pl script
and add linux-rockchip mailing list.

Reported-by: Johan Jonker <jbx6244@gmail.com>
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>