gpio: sim: replace memmove() + strstrip() with skip_spaces() + strim()

Turns out we can avoid the memmove() by using skip_spaces() and strim().
We did that in gpio-consumer, let's do it in gpio-sim.

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