gpio: max77650: Add GPIO support

Add GPIO support for max77650 mfd device. This PMIC exposes a single
GPIO line.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>