MAINTAINERS: Add MIPS-related docs and configs to the MIPS architecture section

docs/system/target-mips.rst and configs/targets/mips* are not covered
in our MAINTAINERS file yet, so let's add them now.

Message-Id: <20221212171252.194864-1-thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>

MAINTAINERS

@@ -113,6 +113,8 @@ M: Philippe Mathieu-Daudé <philmd@linaro.org>
 R: Jiaxun Yang <jiaxun.yang@flygoat.com>
 S: Odd Fixes
 K: ^Subject:.*(?i)mips
+F: docs/system/target-mips.rst
+F: configs/targets/mips*
 
 Guest CPU cores (TCG)
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