Input: gpio_keys_polled - switch to using devm_fwnode_gpiod_get()

devm_fwnode_get_gpiod_from_child() is going away as the name is too
unwieldy, let's switch to using the new devm_fwnode_gpiod_get().

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>