Update version for v6.1.0-rc1 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index 7d5e1d8..f51337e 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-6.0.90
+6.0.91