dt-bindings: pinctrl: at91-pio4: add slew-rate

Document slew-rate DT binding for SAMA7G5.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1611747945-29960-2-git-send-email-claudiu.beznea@microchip.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>