zynq: Add zynq_zybo board support

Zybo is an entry-level embedded software and digital circuit development platform
built around the smallest member of the Xilinx Zynq-7000 family, the Z-7010.

APSOC:
- ZYNQ XC7Z2010-1CLG400C
Memory:
- 512MB x32 DDR3 w/ 1050Mbps bandwidth
- 128Mb Serial Flash w/ QSPI interface
- MicroSD slot (supports Linux file system)
- External EEPROM (programmed with 48-bit globally unique EUI-48/64™ compatible identifier)
Communication:
- Trimode (1Gbit/100Mbit/10Mbit) Ethernet PHY
- OTG USB 2.0 PHY (supports host and device)
- On-board UART to USB converter (shared with jtag programming port)
Multimedia:
- Dual-role (Source/Sink) HDMI port
- 16-bits per pixel VGA output port
- Audio codec with headphone out, microphone and line in jacks
User IO:
- GPIO: 6 pushbuttons, 4 slide switches, 5 LEDs
- Six Pmod connectors (1 processor-dedicated, 1 dual analog/digital)
Misc:
- On-board USB JTAG programming port
- 50 MHz Oscillator

For more information - http://www.digilentinc.com

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