ARM: at91/dt: sama5d4: add pioD controller

PioD controller was not described in the device tree since we don't use
it. As pinctrl-at91 allows disabled gpio controllers in the device
tree, we can add it to complete the device description.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>