MAINTAINERS: Volunteer to maintain Darwin-based hosts support

While I am not an experienced Darwin OS user, I now have to
use a macOS based workstation and alike CI, meaning I should
easily spot regressions and test fixes. I therefore volunteer
to collect Darwin related patches and keep QEMU in good state
on macOS, and to some extent iOS.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Christian Schoenebeck <qemu_oss@crudebyte.com>
Cc: Akihiko Odaki <akihiko.odaki@gmail.com>
Cc: Joelle van Dyne <j@getutm.app>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>