zynq-common: Rename zynq with zynq-common

zynq.h -> zynq-common.h, zynq-common is Common
configuration options for all Zynq boards.

zynq.h is no longer exists hense removed from boards.cfg

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>