Update version for v5.2.0-rc1 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index 167c7a1..b1eec09 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-5.1.90
+5.1.91