gpio: regmap: use new regmap_might_sleep()

Now that the regmap can be queried whether it might sleep, we can get
rid of the conservative setting "can_sleep = true". New drivers which
want to use gpio-regmap and can access the registers memory-mapped won't
have the restriction that their consumers have to use the
gpiod_*cansleep() variants anymore.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>