docs/barrier.txt has a couple of TODO notes about things to be
implemented in this device; move them into a comment in the
source code.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20210727204112.12579-3-peter.maydell@linaro.org