spl: fit: Add support for loading FPGA bitstream

Add support for loading FPGA into the SPL fitImage support. The
mechanism is flexible and allows user to override the actual
function for loading the FPGA itself. This is because on some
systems, the FPGA must be programmed to allow DRAM access, so
loading the full fitImage may not be possible if it contains
the bitstream. Instead, the spl_load_fpga_image() provides all
the tools to load the bitstream in parts while programming it
into the FPGA.

Signed-off-by: Marek Vasut <marex@denx.de>
[trini: Don't always have a branch to print out type]
Signed-off-by: Tom Rini <trini@konsulko.com>