zynq: Add zynq_zc770 xm012 board support

ZC770 is a complete development board based on the Xilinx Zynq-7000
All Programmable SoC, similar to ZC70x board but which has four
different daughter cards, like XM010, XM011, XM012 and XM013

ZC770 XM012:
- 1GB DDR3
- 64MiB Numonyx NOR flash
- USB-UART

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Cc: Stefan Roese <sr@denx.de>