spl: mmc: Support OP-TEE payloads in Falcon mode

spl: mmc: Support OP-TEE payloads in Falcon mode

In general, Falcon mode means we're booting a linux kernel directly.
With FIT images, however, an OP-TEE secure kernel can be booted before
linux. Thus, if the next stage is an IH_OS_TEE, this isn't necessarily
a problem.

Of course, a general solution would involve mmc_load_image_raw_os()
only loading the binary, and leaving the decision of suitability to
someone else. However, a rework of the boot flow is beyond the scope
of this patch. Accept IH_OS_TEE as a valid OS value.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>