fpga: Added support to load bit stream from SD/MMC

Added support to load a bitstream image in chunks by reading it in
chunks from SD/MMC.
Command format:
loadfs [dev] [address] [image size] [blocksize] <interface>
       [<dev[:part]>] <filename>
Example: fpga loadfs 0 1000000 3dbafc 4000 mmc 0 fpga.bin

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