ARM: dts: kirkwood: use unique machine name for ds112

Downstream packages like Debian flash-kernel use
/proc/device-tree/model
to determine which dtb file to install.

Hence each dts in the Linux kernel should provide a unique model
identifier.

Commit 2d0a7add ("ARM: Kirkwood: Add support for many Synology NAS
devices") created the new files kirkwood-ds111.dts and kirkwood-ds112.dts
using the same model identifier.

This patch provides a unique model identifier for the
Synology DiskStation DS112.

Fixes: 2d0a7add ("ARM: Kirkwood: Add support for many Synology NAS devices")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>