ARM: zynq: dts: split up device tree

The purpose of the created zynq-7000.dtsi file is to describe the
hardware common to all Zynq 7000-based boards.  Also, get rid of the
zynq-ep107 device tree, since it is not hardware anyone can purchase.

Add a zc702 dts file based on the zynq-7000.dtsi.  Add it to the
dts/Makefile so it is built with the 'dtbs' target.

Signed-off-by: Josh Cartwright <josh.cartwright@ni.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>