firmware: xilinx: Add fpga API's

This Patch Adds fpga API's to support the Bitstream loading
by using firmware interface.

Signed-off-by: Nava kishore Manne <nava.manne@xilinx.com>
Reviewed-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>