arm: dts: zynq: Add Digilent Zybo Z7 board

This add a DTS for the Digilent Zybo Z7 board.
This board is the successor board of Zybo, these are almost the same except
for ps-clk-frequency specifications.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu@cybertrust.co.jp>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>