gpio: sim: mark the GPIO chip as a one that can sleep

Simulated chips use a mutex for synchronization in driver callbacks so
they must not be called from interrupt context. Set the can_sleep field
of the GPIO chip to true to force users to only use threaded irqs.

Fixes: cb8c474e ("gpio: sim: new testing module")
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>