docs: gpio: Add GPIO Aggregator documentation

Document the GPIO Aggregator, and the two typical use-cases.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
Reviewed-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Link: https://lore.kernel.org/r/20200511145257.22970-6-geert+renesas@glider.be
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/Documentation/admin-guide/gpio/index.rst b/Documentation/admin-guide/gpio/index.rst
index a244ba4..ef28386 100644
--- a/Documentation/admin-guide/gpio/index.rst
+++ b/Documentation/admin-guide/gpio/index.rst
@@ -7,6 +7,7 @@
 .. toctree::
     :maxdepth: 1
 
+    gpio-aggregator
     sysfs
 
 .. only::  subproject and html