Documentation: gpio: move gpio-mockup into obsolete section

The gpio-mockup has been obsoleted by the gpio-sim, so relocate its
documentation into the obsolete section of the admin-guide book.
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Documentation/admin-guide/gpio/index.rst

@@ -9,7 +9,6 @@ GPIO
 
     Character Device Userspace API <../../userspace-api/gpio/chardev>
     gpio-aggregator
-    gpio-mockup
     gpio-sim
     Obsolete APIs <obsolete>
 

Documentation/admin-guide/gpio/obsolete.rst

@@ -9,4 +9,5 @@ Obsolete GPIO APIs
 
     Character Device Userspace API (v1) <../../userspace-api/gpio/chardev_v1>
     Sysfs Interface <../../userspace-api/gpio/sysfs>
+    Mockup Testing Module <gpio-mockup>