gpio: sim: add doc file to index file

Include the gpio-sim.rst file in the GPIO index (toc/table of contents).

Quietens this doc build warning:

Documentation/admin-guide/gpio/gpio-sim.rst: WARNING: document isn't included in any toctree

Fixes: b48f6b466e44 ("gpio: sim: new testing module")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>