test: spl: Add a test for the SPI load method

Add test for the SPI load method. This one is pretty straightforward. We
can't enable FIT_EXTERNAL with LOAD_FIT_FULL because spl_spi_load_image
doesn't know the total image size and has to guess from fdt_totalsize. This
doesn't include external data, so loading it will fail.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>