media: hantro: Add linux-rockchip mailing list to MAINTAINERS

The linux-rockchip mailing list is relevant for the
Hantro driver, given this support the VPU present
in Rockchip SoCs.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>