MAINTAINERS: add co-maintainer for TCG Plugins

I would like to help Alex to maintain TCG Plugins. We talked about it
and he will keep the priority for merging series, but I can occasionally
merge some of them when there is a special interest.

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20260115202511.2709767-1-pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>