docs/system/deprecated: Mark the 'moxie' CPU as deprecated

docs/system/deprecated: Mark the 'moxie' CPU as deprecated

It is currently unclear whether anybody is still using the 'moxie' CPU,
and there are no images for testing available this CPU, so the code has
likely bit-rotten in the course of time. When I asked the maintainer
for information, I did not get a reply within four weeks yet (see
https://lists.gnu.org/archive/html/qemu-devel/2020-08/msg07201.html).
The last Signed-off-by line from Anthony in our repo is from 2013,
so it seems like this code is rather unmaintained. Time to put it onto
the deprecation list to see whether somebody is still interested in this
code or whether we could remove it in a couple of releases.

Message-Id: <20200923171815.97801-1-thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>