ARM: dts: kirkwood: add kirkwood-ds112.dtb to Makefile

Commit 2d0a7add ("ARM: Kirkwood: Add support for many Synology
NAS devices") created the new file kirkwood-ds112.dts but did not
add it to the Makefile.

Fixes: 2d0a7add

 ("ARM: Kirkwood: Add support for many Synology NAS devices")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>