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: NHeinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: NGregory CLEMENT <gregory.clement@free-electrons.com>

arch/arm/boot/dts/Makefile

@@ -215,6 +215,7 @@ dtb-$(CONFIG_MACH_KIRKWOOD) += \
 	kirkwood-ns2mini.dtb \
 	kirkwood-nsa310.dtb \
 	kirkwood-nsa310a.dtb \
+	kirkwood-nsa320.dtb \
 	kirkwood-nsa325.dtb \
 	kirkwood-openblocks_a6.dtb \
 	kirkwood-openblocks_a7.dtb \