arm: xilinx: Use device tree which can be passed on cmd line

DEVICE_TREE variable has been added by commit 6ab6b2afa091
("dts: re-write dts/Makefile more simply with Kbuild")
where DT can be specified on command line. If it is not then
CONFIG_DEFAULT_DEVICE_TREE is used to ensure backward compatibility.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/board/xilinx/zynq/Makefile b/board/xilinx/zynq/Makefile
index 8d33015..6a2acee 100644
--- a/board/xilinx/zynq/Makefile
+++ b/board/xilinx/zynq/Makefile
@@ -14,7 +14,7 @@
 endif
 
 ifeq ($(init-objs),)
-hw-platform-y :=$(shell echo $(CONFIG_DEFAULT_DEVICE_TREE))
+hw-platform-y :=$(shell echo $(DEVICE_TREE))
 init-objs := $(if $(wildcard $(srctree)/$(src)/$(hw-platform-y)/ps7_init_gpl.c),\
 	$(hw-platform-y)/ps7_init_gpl.o)
 endif
diff --git a/board/xilinx/zynqmp/Makefile b/board/xilinx/zynqmp/Makefile
index 5ace6cc..174f4ed 100644
--- a/board/xilinx/zynqmp/Makefile
+++ b/board/xilinx/zynqmp/Makefile
@@ -14,7 +14,7 @@
 endif
 
 ifeq ($(init-objs),)
-hw-platform-y :=$(shell echo $(CONFIG_DEFAULT_DEVICE_TREE))
+hw-platform-y :=$(shell echo $(DEVICE_TREE))
 init-objs := $(if $(wildcard $(srctree)/$(src)/$(hw-platform-y)/psu_init_gpl.c),\
 	$(hw-platform-y)/psu_init_gpl.o)
 endif