mailmap: add entry for Tiezhu Yang

Add an entry to connect all my email addresses.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Link: https://lore.kernel.org/r/1580721045-4988-1-git-send-email-yangtiezhu@loongson.cn


Signed-off-by: Jonathan Corbet <corbet@lwn.net>