spl: socfpga: Implement fpga bitstream loading with socfpga loadfs

Add support for loading FPGA bitstream to get DDR up running before
U-Boot is loaded into DDR. Boot device initialization, generic firmware
loader and SPL FAT support are required for this whole mechanism to work.

Signed-off-by: Tien Fong Chee <tien.fong.chee@intel.com>