pinctrl: at91: add support for OUTPUT config

Add support for pin output control through the pinctrl config:
 - support enabling/disabling output on a given pin
 - support output level setting (high or low)

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>