MAINTAINERS: add the "EDK2 Firmware" subsystem

We now have the edk2 submodule, somewhat elaborate build helpers for it,
and even a UEFI application written against edk2 whose genuine home is the
QEMU repository. Add the "EDK2 Firmware" subsystem such that all relevant
pathnames be covered.

Suggested-by: Daniel P. Berrang <berrange@redhat.com>
Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Suggested-by: Philippe Mathieu-Daud <philmd@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Philippe Mathieu-Daud <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daud <philmd@redhat.com>
Tested-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>