gpio: fxl6408: add I2C GPIO expander driver

Add minimal driver for Fairchild FXL6408 8-bit I2C-controlled GPIO expander
using the generic regmap based GPIO driver (GPIO_REGMAP).

The driver implements setting the GPIO direction, reading inputs
and writing outputs.

In addition to that the FXL6408 has the following functionalities:
- allows to monitor input ports for data transitions with an interrupt pin
- all inputs can be configured with pull-up or pull-down resistors

Datasheet: https://www.onsemi.com/download/data-sheet/pdf/fxl6408-d.pdf


Signed-off-by: Emanuele Ghidoli <emanuele.ghidoli@toradex.com>
Co-developed-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
[Bartosz: order includes alphabetically]
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>