arm64: dts: prevent *.dtb from always being rebuilt

if_changed (used by the *.dts->*.dtc rule) rebuilds files if they aren't
contained in $(targets). (make V=2 indicates this). Add $(dtb-y) to
$(targets) to prevent *.dtb from always being rebuilt. Note

This fixes a regression introduced by the .dtb rule rework in da4cbc6
"arm64: use new common dtc rule", although since arm64 doesn't actually
have any *.dts yet, this isn't a critical issue.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
diff --git a/arch/arm64/boot/dts/Makefile b/arch/arm64/boot/dts/Makefile
index 801e2d7..32ac0ae 100644
--- a/arch/arm64/boot/dts/Makefile
+++ b/arch/arm64/boot/dts/Makefile
@@ -1,4 +1,5 @@
 targets += dtbs
+targets += $(dtb-y)
 
 dtbs: $(addprefix $(obj)/, $(dtb-y))