ARM: at91: gpio: implement request

Configure the pin as pio when requested.

It is needed to configure the pin as PIO at "request time" when we are
using DT. Indeed, the muxing via old AT91 API is not allowed anymore if
we are using the plain gpiolib.

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>