docs: gpio: add sysfs interface to the admin-guide

While this is stated as obsoleted, the sysfs interface described
there is still valid, and belongs to the admin-guide.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/ABI/obsolete/sysfs-gpio

@@ -11,7 +11,7 @@ Description:
   Kernel code may export it for complete or partial access.
 
   GPIOs are identified as they are inside the kernel, using integers in
-  the range 0..INT_MAX.  See Documentation/gpio for more information.
+  the range 0..INT_MAX.  See Documentation/admin-guide/gpio for more information.
 
     /sys/class/gpio
 	/export ... asks the kernel to export a GPIO to userspace

Documentation/admin-guide/gpio/index.rst

@@ -1,4 +1,4 @@
-:orphan:
+.. SPDX-License-Identifier: GPL-2.0
 
 ====
 gpio

Documentation/admin-guide/index.rst

@@ -91,6 +91,7 @@ configure specific aspects of kernel behavior to your liking.
    cputopology
    device-mapper/index
    efi-stub
+   gpio/index
    highuid
    hw_random
    iostats

Documentation/firmware-guide/acpi/enumeration.rst

@@ -316,7 +316,7 @@ specifies the path to the controller. In order to use these GPIOs in Linux
 we need to translate them to the corresponding Linux GPIO descriptors.
 
 There is a standard GPIO API for that and is documented in
-Documentation/gpio/.
+Documentation/admin-guide/gpio/.
 
 In the above example we can get the corresponding two GPIO descriptors with
 a code like this::

Documentation/translations/zh_CN/gpio.txt

@@ -1,4 +1,4 @@
-Chinese translated version of Documentation/gpio
+Chinese translated version of Documentation/admin-guide/gpio
 
 If you have any comment or update to the content, please contact the
 original document maintainer directly.  However, if you have a problem
@@ -10,7 +10,7 @@ Maintainer: Grant Likely <grant.likely@secretlab.ca>
 		Linus Walleij <linus.walleij@linaro.org>
 Chinese maintainer: Fu Wei <tekkamanninja@gmail.com>
 ---------------------------------------------------------------------
-Documentation/gpio 的中文翻译
+Documentation/admin-guide/gpio 的中文翻译
 
 如果想评论或更新本文的内容，请直接联系原文档的维护者。如果你使用英文
 交流有困难的话，也可以向中文版维护者求助。如果本翻译更新不及时或者翻

MAINTAINERS

@@ -6867,7 +6867,7 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/linusw/linux-gpio.git
 S:	Maintained
 F:	Documentation/devicetree/bindings/gpio/
 F:	Documentation/driver-api/gpio/
-F:	Documentation/gpio/
+F:	Documentation/admin-guide/gpio/
 F:	Documentation/ABI/testing/gpio-cdev
 F:	Documentation/ABI/obsolete/sysfs-gpio
 F:	drivers/gpio/