test: doc: Add documentation for ut command

Before adding more options, document this command.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index df50746..f3c8fba 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -74,6 +74,7 @@
    cmd/tftpput
    cmd/true
    cmd/ums
+   cmd/ut
    cmd/wdt
    cmd/xxd