gpio: mockup: use pr_fmt()

We don't need a custom logging helper. Let's use the standard pr_fmt()
macro which allows us to use all pr_*() routines with custom format.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>