Update version for v9.2.0-rc1 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index 46b58d6..91065aa 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-9.1.90
+9.1.91