doc: Add usage information for the acpi command

Add some documentation for this command.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 33761af..964d761 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -18,6 +18,7 @@
 .. toctree::
    :maxdepth: 1
 
+   acpi
    addrmap
    askenv
    base