doc: Add some general information about the ACPI subsystem

The ACPI subsystem is poorly documented outside the code iitself. Add
some general information, focussed mainly on how to add the ACPI
tables for a new board into U-Boot.

Series-to: concept
Series-cc: Jaiprakash Singh <jaiprakashs@marvell.com>

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Jaiprakash Singh <jaiprakashs@marvell.com>

doc/develop/index.rst

@@ -27,9 +27,10 @@ Implementation
 --------------
 
 .. toctree::
-   :maxdepth: 1
+   :maxdepth: 4
 
    directories
+   acpi
    bloblist
    bootstd/index
    ci_testing