ARM: dts: kirkwood: add kirkwood-nsa320.dtb to Makefile

Commit be3d7d02 ("ARM: kirkwood: Add DTS file for NSA320")
created the new file kirkwood-nsa320.dts but did not
add it to the Makefile.

Fixes: be3d7d02

 ("ARM: kirkwood: Add DTS file for NSA320")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>