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 da4cbc6d
"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>