gpio: mark unsafe gpio_chip manipulators as deprecated

We still have some functions that return the address of the GPIO chip
associated with the GPIO device. This is dangerous and the users should
find a better solution. Let's add appropriate comments to the kernel
docs.

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