fpga: Add support to load partial bitstreams

Added support to load partial bitstreams.
The partial bitstreams can be loaded using the below commands
Commands:
fpga loadp <dev> <addr> <size>
fpga loadbp <dev> <addr> <size>
The full bit streams can be loaded using the
old commands(fpga load and fpga loadb).

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