Add Yanteng Si to .mailmap

Yanteng Si's old address bounces, creating a lot of noise in conversations
about patches.  Add a .mailmap entry so scripts/get_maintainer.pl will
return a working address.

Acked-by: Yanteng Si <si.yanteng@linux.dev>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/87plny1zam.fsf@trenco.lwn.net

Author: Jonathan Corbet

.mailmap

@@ -717,6 +717,7 @@ Will Deacon <will@kernel.org> <will.deacon@arm.com>
 Wolfram Sang <wsa@kernel.org> <w.sang@pengutronix.de>
 Wolfram Sang <wsa@kernel.org> <wsa@the-dreams.de>
 Yakir Yang <kuankuan.y@gmail.com> <ykk@rock-chips.com>
+Yanteng Si <si.yanteng@linux.dev> <siyanteng@loongson.cn>
 Yusuke Goda <goda.yusuke@renesas.com>
 Zack Rusin <zack.rusin@broadcom.com> <zackr@vmware.com>
 Zhu Yanjun <zyjzyj2000@gmail.com> <yanjunz@nvidia.com>