spl: Convert spl_ram_load_image() to use linker list

Add a linker list declaration for this method and remove the explicit
switch() code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
[trini: Include updating the DFU case]
Signed-off-by: Tom Rini <trini@konsulko.com>