spl: spi: Consolidate spi_load_image_os into spl_spi_load_image

spi_load_image_os performs almost the same steps as the non-falcon-boot
path of spl_spi_load_image. The load address is different, and it also
loads a device tree, but that's it. Refactor the boot process so that
they can both use the same load function.

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