Doc/ioexpand: add descriptions about ioexpander

Signed-off-by: buxiasen <[email protected]>

Documentation/components/drivers/special/ioexpander.rst

@@ -1,3 +1,35 @@
 ==========================
 IO Expander Device Drivers
 ==========================
+
+-  ``include/nuttx/ioexpander/ioexpander.h`` and ``include/nuttx/ioexpander/gpio.h``.
+   All structures and APIs needed to work with ioexpander drivers are provided in
+   this header file.
+
+-  ``struct ioexpander_ops_s``. Each ioexpand device driver must implement
+   an instance of ``struct ioexpander_ops_s``. That structure defines a
+   call table with the methods, and we also provide macros to help access methods.
+
+-  we also provide method ``gpio_lower_half`` to make ioexpander compatible with normal gpio.
+
+-  **Binding ioexpander Drivers**. ioexpander drivers are not normally directly
+   accessed by user code, we should always get lower level drivers, for example I2C,
+   and map extended gpio feature same asa normal gpio. See for example,
+   ``int nrf52_sx1509_initialize(void)``
+   in ``boards/arm/nrf52/thingy52/src/nrf52_sx1509.c``. In general, the binding
+   sequence is:
+
+   #. Get an instance of ``struct i2c_master_s`` from the
+      hardware-specific I2C device driver, and
+   #. Provide that instance and configurations to the ioexpander initialization method
+      to get the ``struct ioexpander_dev_s`` ioe device instance.
+   #. Then use ioe device instance to do ioexpander operations, or use ``gpio_lower_half``
+      to make ioexpand compatible with normal gpio.
+
+
+-  **Examples**: ``drivers/ioexpander/pca9555.c``,
+   ``drivers/input/aw86225.c``,
+   ``drivers/analog/lmp92001.c``,
+   ``drivers/ioexpander/ioe_rpmsg.c``,
+   ``boards/sim/sim/sim/src/sim_ioexpander.c``,
+   ``boards/arm/nrf52/thingy52/src/nrf52_sx1509.c`` etc.