[mips] Add LiuYu to MIPS_OWNERS

I'm one of the v8 maintainers of mips, and I request to add
myself to MIPS_OWNERS.

Change-Id: If246f44e54497619d9d58929131e3d2d1dfc9c5b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3141031
Auto-Submit: Liu yu <liuyu@loongson.cn>
Reviewed-by: Lutz Vahl <vahl@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Liu yu <liuyu@loongson.cn>
Cr-Commit-Position: refs/heads/main@{#76714}

MIPS_OWNERS

 xwafish@gmail.com
 zhaojiazhong-hf@loongson.cn
+liuyu@loongson.cn