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 <[e-mail]>
Signed-off-by: Bartosz Golaszewski <[e-mail]>

Documentation/admin-guide/gpio/index.rst

···
 
     Character Device Userspace API <../../userspace-api/gpio/chardev>
     gpio-aggregator
-    gpio-mockup
     gpio-sim
     Obsolete APIs <obsolete>
 

Documentation/admin-guide/gpio/obsolete.rst

···
 
     Character Device Userspace API (v1) <../../userspace-api/gpio/chardev_v1>
     Sysfs Interface <../../userspace-api/gpio/sysfs>
+    Mockup Testing Module <gpio-mockup>