Update version for v6.0.0-rc1 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index 9b9d017..298bd24 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-5.2.90
+5.2.91