gpio: fx6408: Convert to use maple tree register cache

The maple tree register cache is based on a much more modern data structure
than the rbtree cache and makes optimisation choices which are probably
more appropriate for modern systems than those made by the rbtree cache.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

gpio: Switch i2c drivers back to use .probe()

After commit b8a1a4cd5a98 ("i2c: Provide a temporary .probe_new()
call-back type"), all drivers being converted to .probe_new() and then
03c835f498b5 ("i2c: Switch .probe() to not take an id parameter") convert
back to (the new) .probe() to be able to eventually drop .probe_new() from
struct i2c_driver.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

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>