Deprecate Unicore32 port

Deprecate our Unicore32 target support:
 * the Linux kernel dropped support for unicore32 in commit
   05119217a9bd199c for its 5.9 release (with rationale in the
   cover letter: https://lkml.org/lkml/2020/8/3/232 )
 * there is apparently no upstream toolchain that can create unicore32
   binaries
 * the maintainer doesn't seem to have made any contributions to
   QEMU since the port first landed in 2012
 * nobody else seems to have made changes to the unicore code except
   for generic cleanups either

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20200825172719.19422-1-peter.maydell@linaro.org

docs/system/deprecated.rst

@@ -408,6 +408,14 @@ The above, converted to the current supported format::
 linux-user mode CPUs
 --------------------
 
+``unicore32`` CPUs (since 5.2.0)
+''''''''''''''''''''''''''''''''
+
+The ``unicore32`` guest CPU support is deprecated and will be removed in
+a future version of QEMU. Support for this CPU was removed from the
+upstream Linux kernel, and there is no available upstream toolchain
+to build binaries for it.
+
 ``tilegx`` CPUs (since 5.1.0)
 '''''''''''''''''''''''''''''