image-fit: Accept OP-TEE images when booting a FIT

OP-TEE images are normally packaged with
	type = "tee;
	os = "tee";

However, fit_image_load() thinks that is somehow invalid. However if
they were declared as type = "kernel", os = "linux", fit_image_load()
would happily accept them and allow the boot to continue. There is no
technical limitation to excluding "tee".

Allowing "tee" images is useful in a boot flow where OP-TEE is
executed before linux.

In fact, I think it's unintuitive for a "load"ing function to also do
parsing and contain a bunch ad-hoc heuristics that only its caller
might know. But I don't make the rules, I just write fixes. In more
polite terms: refactoring the fit_image API is beyond the scope of
this change.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>