ARM: at91/at91-pinctrl documentation: fix spelling mistake: "contoller" -> "controller"

trivial fix to spelling mistake in documentation

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>