ARM: dts: kirkwood: Fix Openblock A6 external GPIO pins

By assigning the pin hog to the pinctrl node, we correctly configure the
MPPs.  However, they are not available to userspace.

Fix this by assigning the hogs to the gpio node.

After this, the following works as expected:

  # echo 28 >/sys/class/gpio/export
  # echo low >/sys/class/gpio/gpio28/direction

[gregory.clement@free-electrons.com: fix title]
Signed-off-by: Jason Cooper <jason@lakedaemon.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>