arm: xilinx: Fill DEVICE_TREE directly in Makefiles

DEVICE_TREE variable is not exported from dts/Makefile that's why it is
necessary to setup directly before use.

Fixes: 312f2c5b ("arm: xilinx: Use device tree which can be passed on cmd line")
Signed-off-by: Michal Simek <michal.simek@xilinx.com>