MAINTAINERS: Add docs/system/i386/ to the general x86 architecture section

MAINTAINERS: Add docs/system/i386/ to the general x86 architecture section

We've got a section for generic x86 architecture support in our
MAINTAINERS file - this should cover the docs/system/i386/ folder, too.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>