fpga: Add support for gzip images with bitstreams

Here is the set of command which has been performed
to proof this feature.

gzip < fpga.bin > fpga.bin.gz
mkimage -A arm -O u-boot -T firmware -C gzip \
-a 20000000 -n "zc702_fpga_bin" -d fpga.bin.gz fpga.bin.gz.ub

tftp 100000 fpga.bin.gz.ub
fpga loadmk 0 100000

This flow should speedup loading bitstream data
from external memory and save image footprint in non volatile
memory.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>