spl: legacy: Fix referencing _image_binary_end

On non-arm architectures, _image_binary_end is defined as a ulong and not a
char[]. Take the address of it when accessing it, which is correct for
both.

Fixes: 1b8a1be1 ("spl: spl_legacy: Fix spl_end address")
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>