arm: zynq: add support for the zybo z7 board

arm: zynq: add support for the zybo z7 board

The board is manufactured by Digilent
Main features:
- Soc: XC7Z010 (Z7-10) or XC7Z020 (Z7-20)
- RAM: 1 GB DDR3L
- FLASH: 16 MB QSPI
- 1 Gbps Ethernet
- USB 2.0
- microSD slot
- Pcam camera connector
- HDMI Tx and Rx
- Audio codec: stereo out, stereo in, mic
- 5 (Z7-10) or 6 (Z7-20) Pmod ports
- 6 push-buttons, 4 switches, 5 LEDs
- 1 (Z7-10) or 2 (Z7-20) RGB LEDs

Signed-off-by: Luis Araneda <luaraneda@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>