cmd: rng: Add rng list command

The 'rng list' command probes all RNG devices and list those devices
that are successfully probed. Also update the help info.

Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Weizhao Ouyang <o451686892@gmail.com>
diff --git a/doc/usage/cmd/rng.rst b/doc/usage/cmd/rng.rst
index 274e4d8..4a61e33 100644
--- a/doc/usage/cmd/rng.rst
+++ b/doc/usage/cmd/rng.rst
@@ -11,16 +11,22 @@
 
 ::
 
-    rng [devnum [n]]
+    rng list
+    rng [dev] [n]
 
-Description
------------
+rng list
+--------
+
+List all the probed rng devices.
+
+rng [dev] [n]
+-------------
 
 The *rng* command reads the random number generator(RNG) device and
 prints the random bytes read on the console. A maximum of 64 bytes can
 be read in one invocation of the command.
 
-devnum
+dev
     The RNG device from which the random bytes are to be
     read. Defaults to 0.