dts: zynq: Add basic fdt support

This patch provides a basic fdt support for zynq u-boot.

zynq-7000.dtsi-> initial arch dts file
zynq-zed.dts -> initial zed board dts file
more devices should be added in subsequent patches.

u-boot build: once configuring of a board done
for building dtb with zynq-zed.dts as an input
zynq-uboot> make DEVICE_TREE=zynq-zed

Enabled CONFIG_OF_SEPARATE for building dtb separately.
There is a new binary called u-boot-dtb.bin which is a u-boot
with devicetree supported.

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>