ARM: dts: kirkwood: Add DTS for Zyxel NSA325

Add a new DTS file to support the Zyxel NSA325(v2) dual bay
NAS device, based on the NSA320 DTS files.

The only difference to the NSA320 device is GPIO47.
This en/disables the power for the hdd in slot2, currently
fixed to on.

[gregory.clement@free-electrons.com: fix comment format]
Signed-off-by: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>