Update version for v2.3.0-rc1 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

VERSION

diff --git a/VERSION b/VERSION
index 6f898f5..1a848c1 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.2.90
+2.2.91