gpiolib: rename static functions that are called with the lock taken

Rename two functions that read or modify the global GPIO device list but
don't take the lock themselves (and need to be called with it already
acquired). Use the _unlocked() suffix which seems to be used quite
consistently across the kernel despite there also existing the _locked()
suffix for the same purpose.

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