Update version for v2.12.0-rc1 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index 9ecd15d..5610569 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.11.90
+2.11.91