pinctrl: at91-pio4: add support for slew-rate

SAMA7G5 supports slew rate configuration. Adapt the driver for this.
For output switching frequencies lower than 50MHz the slew rate needs to
be enabled. Since most of the pins on SAMA7G5 fall into this category
enabled the slew rate by default.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Link: https://lore.kernel.org/r/1611747945-29960-3-git-send-email-claudiu.beznea@microchip.com


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