MAINTAINERS: Cover the firmware JSON schema

MAINTAINERS: Cover the firmware JSON schema

Add an entry to cover firmware.json (see commit 3a0adfc9bf:
schema that describes the different uses and properties of
virtual machine firmware).

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Kashyap Chamarthy <kchamart@redhat.com>
Cc: Daniel P. Berrange <berrange@redhat.com>
Suggested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200703183450.32398-1-philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Kashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>