gpio: sim: Initialize attribute allocated on the heap

gpio: sim: Initialize attribute allocated on the heap

The attributes on the heap must be initialized before use.
Neglecting that will produce an Oops in some configurations:

  BUG: key ffff000800eba398 has not been registered!

Initialize attribute allocated on the heap.

Fixes: 3f0279eb9e37 ("gpio: sim: new testing module")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>