ARM: 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.

This fixes a regression introduced by the .dtb rule rework in 499cd829
"ARM: dt: change .dtb build rules to build in dts directory".
Reported-by: NShawn Guo <shawn.guo@linaro.org>
Signed-off-by: NStephen Warren <swarren@nvidia.com>
Signed-off-by: NRob Herring <rob.herring@calxeda.com>

arch/arm/boot/dts/Makefile

@@ -155,6 +155,7 @@ dtb-$(CONFIG_ARCH_VT8500) += vt8500-bv07.dtb \
 dtb-$(CONFIG_ARCH_ZYNQ) += zynq-zc702.dtb
 
 targets += dtbs
+targets += $(dtb-y)
 endif
 
 # *.dtb used to be generated in the directory above. Clean out the