MAINTAINERS: erofs: complete sub-entries for erofs

Add a formal git tree and missing files for erofs
after moving out of staging for everyone to blame
in order for better improvement.

Acked-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20190919062838.106423-1-gaoxiang25@huawei.com
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>