gpio: mockup: use device properties instead of platform_data

Some users want to introduce device tree support to the mockup driver.
Let's make it easier by switching to using generic device properties.
The driver stays compatible with previous use cases and after this
conversion there'll be no need to change the way probing of mockup
GPIO chips works.

Tested with libgpiod test suite.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>