auxdisplay: Add I2C gpio expander example

The hd44780 displays are often used with pcf8574 based I/O expanders.
Add example to documentation.

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Ralf Schlatterbeck <rsc@runtux.com>
[Added Suggested-by tag]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>