gpiolib: Split out for_each_gpio_desc() macro

In some cases we want to traverse all GPIO descriptors for given
chip, let's split out for_each_gpio_desc() macro for such cases.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>