hw/arm/highbank: Mark the "highbank" and the "midway" machine as deprecated

hw/arm/highbank: Mark the "highbank" and the "midway" machine as deprecated

We don't have any automatic regression tests for these machines and
when asking the usual suspects on the mailing list we came to the
conclusion that nobody tests these machines manually, too, so it seems
like this is currently just completely unused code. Mark them as depre-
cated to see whether anybody still speaks up during the deprecation
period, otherwise we can likely remove these two machines in a couple
of releases.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Message-id: 20250702113051.46483-1-thuth@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: tweaked deprecation.rst text]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>