ARM: Kirkwood: fix ns2 gpios by converting to pinctrl

Note that the pinctrl conversion also fixes GPIO support for ns2 boards.
Since commit f9e75922

: "ARM: Kirkwood: Make use of mvebu pincltl and
gpio", the mvbu_gpio driver is used for DT boards. As mvbu_gpio relies
on the pinctrl driver, then a pinctrl definition must be given to allow
the GPIO configuration.

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>