x86: Add a cpuid command

It is useful to obtain the results of cpuid queries, so add a command
for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index fcce125..d2b187b 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -52,6 +52,7 @@
    cmd/conitrace
    cmd/cp
    cmd/cpu
+   cmd/cpuid
    cmd/cyclic
    cmd/dm
    cmd/ebtupdate