gpiolib: Consolidate the allocated mask freeing APIs

There is a common API to allocate a mask, but more than one duplicative
counterparts. Consolidate the latter into a single common API beneath.

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>