gpio: max77620: use the new open drain callback

The MAX77620 have a GPIO pins which can act as open drain or
push pull mode. Implement support for controlling this from GPIO
descriptor tables or other hardware descriptions such as
device tree by implementing the .set_single_ended() callback.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>