dts: gpio_atmel: adapt binding doc to reality

The second cell in a gpio reference is used to pass GPIO_ACTIVE_LOW or
GPIO_ACTIVE_HIGH. The gpio device can also be used as irq controller and
a reference can contain the IRQ_TYPE_* values in the second cell.

Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>