Update version for v5.2.0-rc2 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index b1eec09..f01420a 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-5.1.91
+5.1.92