docs: driver-api: Add GPIO section

This adds a section about the various parts of the GPIO subsystem to the
driver API documentation. Note that this isn't exhaustive documentation,
but rather focusses on pulling in the kerneldoc from various sources, in
order to improve coverage of kerneldoc processing.
Signed-off-by: NThierry Reding <treding@nvidia.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

Documentation/driver-api/gpio.rst

+===================================
+General Purpose Input/Output (GPIO)
+===================================
+
+Core
+====
+
+.. kernel-doc:: include/linux/gpio/driver.h
+   :internal:
+
+.. kernel-doc:: drivers/gpio/gpiolib.c
+   :export:
+
+Legacy API
+==========
+
+The functions listed in this section are deprecated. The GPIO descriptor based
+API described above should be used in new code.
+
+.. kernel-doc:: drivers/gpio/gpiolib-legacy.c
+   :export:
+
+ACPI support
+============
+
+.. kernel-doc:: drivers/gpio/gpiolib-acpi.c
+   :export:
+
+Device tree support
+===================
+
+.. kernel-doc:: drivers/gpio/gpiolib-of.c
+   :export:
+
+Device-managed API
+==================
+
+.. kernel-doc:: drivers/gpio/devres.c
+   :export:
+
+sysfs helpers
+=============
+
+.. kernel-doc:: drivers/gpio/gpiolib-sysfs.c
+   :export:

Documentation/driver-api/index.rst

@@ -44,6 +44,7 @@ available subsections can be seen below.
    uio-howto
    firmware/index
    pinctl
+   gpio
    misc_devices
 
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