spl: change load_addr and entry_point to uintptr_t

Mainly a stylistic change: convert the load_addr and entry_point
fields of struct spl_image_info to uintptr_t (from ulong).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>