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

DEVICE_TREE variable has been added by commit 6ab6b2af
("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>