arm: zynq: Add support for zybo board

The ZYBO (Zynq Board) is a feature-rich, ready-to-use, entry-level
embedded software and digital circuit development platform built around
the smallest member of the Xilinx Zynq-7000 family, the Z-7010.

The device tree is similar to zed board, with the clock frequency changed
from 33.33MHz to 50MHz.

Signed-off-by: Tinghui Wang <steven.wang@digilentinc.com>