fpga: Add flag to indicate bitstream needs decompression

Add a flag that is passed to the write_init() callback, indicating
that the bitstream is compressed.

The low-level driver will deal with the flag, or return an error,
if compressed bitstreams are not supported.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>