doc: rng: Add documentation for the rng command

Add a usage document for the 'rng' u-boot command.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 16a3db5..2ba8733 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -56,6 +56,7 @@
    cmd/pstore
    cmd/qfw
    cmd/reset
+   cmd/rng
    cmd/sbi
    cmd/sf
    cmd/scp03