ARM: mvebu: kirkwood: add PogoPlug series 4 device tree

This adds a device tree for the CloudEngines PogoPlug series 4
NAS device. Inspired by out-of-tree boardfiles from ArchLinux
by Kevin Mihelich.

Cc: Moonman <moonman.ca@gmail.com>
Cc: Kevin Mihelich <kevin@archlinuxarm.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>