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 499cd82
"ARM: dt: change .dtb build rules to build in dts directory".

Reported-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>