ath79: drop dts/Makefile

Signed-off-by: John Crispin <john@phrozen.org>

target/linux/ath79/dts/Makefile

-# SPDX-License-Identifier: GPL-2.0
-# All DTBs
-dtb-$(CONFIG_ATH79)			+= ar9132_tl_wr1043nd_v1.dtb
-dtb-$(CONFIG_ATH79)			+= ar9331_dpt_module.dtb
-dtb-$(CONFIG_ATH79)			+= ar9331_dragino_ms14.dtb
-dtb-$(CONFIG_ATH79)			+= ar9331_omega.dtb
-dtb-$(CONFIG_ATH79)			+= ar9331_tl_mr3020.dtb
-
-# Force kbuild to make empty built-in.o if necessary
-obj-				+= dummy.o
-
-always				:= $(dtb-y)
-clean-files			:= *.dtb *.dtb.S