doc: man-page for smbios command

Provide a man-page for the smbios command.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index c171c02..0d174ee 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -103,6 +103,7 @@
    cmd/size
    cmd/sleep
    cmd/sm
+   cmd/smbios
    cmd/sound
    cmd/source
    cmd/temperature