Documentation: gpio: fix bindings document

[ imported from Linux Kernel, commit 74981fb81d83 ]
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Simon Glass <sjg@chromium.org>