pinctrl: dt-binding: Add DT binding documentation for Atmel PIO4

Add documentation for the Atmel PIO4 controller introduced with SAMA5D2
chip family.

Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>