Recently upstreamed changes to OP-TEE mean that it is possible for OP-TEE
to provide a DTB overlay directly to subsequent boot stages thus negating
the requirement to bundle a DTB in the FIP.

This patch switches off the dependency on the DTB in the FIP descriptor
instead we will provide the necessary data as an overlay from OP-TEE.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>