pinctrl: at91: implement at91_pinconf_dbg_show

This allows to get the pin configuration by using debugfs. On my system:
 # cat /sys/kernel/debug/pinctrl/pinctrl.3/pinconf-pins

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>