hw/arm/virt: Declare fwcfg as dma cache coherent in dt

Fw-cfg recently learned how to directly access guest memory and does so in
cache coherent fashion. Tell the guest about that fact when it's using DT.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Shannon Zhao <shannon.zhao@linaro.org>
Message-id: 1486644810-33181-5-git-send-email-agraf@suse.de
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>