Update version for v2.6.0-rc2 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index e710b3f..f313bba 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.5.91
+2.5.92