configs: zynq: Add config support for ZYBO

This patch adds config support needed for ZYBO target and has
been tested on Rev. B ZYBO hardware.

Signed-off-by: Tinghui Wang <steven.wang@digilentinc.com>
[PC changes:
 * Dropped boards.cfg change (rebase conflict)
 * Dropped custom u-boot env
 * Added DTS
 * Misc forward-porting changes
]
Signed-off-by: Peter Crosthwiate <crosthwaite.peter@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>