gpio: pcf857x: Get rid of legacy platform data

Platform data is a legacy interface to supply device properties
to the driver. In this case we don't have in-kernel users for it.
Moreover it uses plain GPIO numbers which is no-no for a new code.

Just remove it for good.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>