pinctrl: at91: Enable slewrate by default on SAM9X60

pinctrl: at91: Enable slewrate by default on SAM9X60

On SAM9X60, slewrate should be enabled on pins with a switching frequency
below 50Mhz. Since most of our pins do not exceed this value, we enable
slewrate by default. Pins with a switching value that exceeds 50Mhz will
have to explicitly disable slewrate.

This patch changes the ABI. However, the slewrate macros are only used
by SAM9X60 and, at this moment, there are no device-tree files available
for this platform.

Suggested-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Link: https://lore.kernel.org/r/20191101092031.24896-1-codrin.ciubotariu@microchip.com
Acked-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>