fpga: Add bitstream type BIT_NONE

Add bitstream type BIT_NONE to the bitstream type
enum. This might be useful while loading bitstreams
in respective drivers.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>