spl: fit: Load devicetree when a Linux payload is found

spl: fit: Load devicetree when a Linux payload is found

When a FIT config specifies a devicetree, we should load it, no
questions asked. In the case of the "simple" FIT loading path, a
difficulty arises in selecting the load address of the FDT.

The default FDT location is right after the "kernel" or "firmware"
image. However, if that is an OP-TEE image, then the FDT may end up in
secure DRAM, and not be accessible to normal world kernels.

Although the best solution is to be more careful about the FDT
address, a viable workaround is to only append the FDT after a u-boot
or Linux image. This is identical to the previous logic, except that
FDT loading is extended to IH_OS_LINUX images.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>