MAINTAINERS: Add include/hw/xtensa/mx_pic.h to the XTFPGA machine section

MAINTAINERS: Add include/hw/xtensa/mx_pic.h to the XTFPGA machine section

These machines are the only user of the mx_pic code, so the
header (which is currently "unmaintained" according to the
MAINTAINERS file) should be added to this section.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20231107102104.14342-1-thuth@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>