gpio: wcd934x: mark the GPIO controller as sleeping

The slimbus regmap passed to the GPIO driver down from MFD does not use
fast_io. This means a mutex is used for locking and thus this GPIO chip
must not be used in atomic context. Change the can_sleep switch in
struct gpio_chip to true.

Fixes: 59c32468 ("gpio: wcd934x: Add support to wcd934x gpio controller")
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>