dualroot: add chain of trust for Platform owned SPs

For dualroot CoT there are two sets of SP certificates, one owned by
Silicon Provider(SiP) and other owned by Platform. Each certificate can
have a maximum of 4 SPs.

This patch reduces the number of SiP owned SPs from 8 to 4 and adds
the remaining 4 to Plat owned SP.
Plat owned SP certificate is signed using Platform RoT key and
protected against anti-rollback using the Non-trusted Non-volatile
counter.

Change-Id: Idc3ddd87d6d85a5506a7435f45a6ec17c4c50425
Signed-off-by: Manish Pandey <manish.pandey2@arm.com>