MAINTAINERS: add maintainers for DRM LOONGSON driver

This patch add Sui Jingfeng as maintainer to drm/loongson driver.

Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230615143613.1236245-3-15330273260@189.cn