docs: Deprecate the pseries-2.12 machines

docs: Deprecate the pseries-2.12 machines

pSeries machines before 3.0 have complex migration back
compatibility code we'd like to get ride of. The last
one is 2.12, which is 6 years old. We just deprecated up
to the 2.11 machine in commit 1392617d35 ("spapr: Tag
pseries-2.1 - 2.11 machines as deprecated").
Take to opportunity to also deprecate the 2.12 machines.

Acked-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>