tbbr: add chain of trust for Secure Partitions

tbbr: add chain of trust for Secure Partitions

with sha 44f1aa8, support for Silicon Provider(SiP) owned Secure
Partition(SP) was added for dualroot CoT. This patch extends this
support for tbbr CoT.

Earlier tbbr CoT for SPs was left to avoid adding new image types in
TBBR which could possibly be seen as deviation from specification.
But with further discussions it is understood that TBBR being a
*minimal* set of requirements that can be extended as long as we don't
violate any of the musts, which is the case with adding SP support.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I1b9e3ebdd7d653f1fd4cc3bd910a69871b55ecbb