zynq-common: Define default environment

Defined default env. for autoboot FIT image from
respective boot devices.

Default settings:
fit_image=fit.itb
load_addr=0x2000000
fit_size=0x800000
flash_off=0x100000
nor_flash_off=0xE2100000

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