Add support to generate certificate "sip-sp-cert" for Secure
Partitions(SP) owned by Silicon provider(SiP).
To avoid deviation from TBBR specification the support is only added for
dualroot CoT and not for TBBR CoT.

A single certificate file is generated containing hash of individual
packages. Maximum 8 secure partitions are supported.

Following new options added to cert_tool:
 --sip-sp-cert --> SiP owned Secure Partition Content Certificate
 --sp-pkg1 --> Secure Partition Package1 file
 --sp-pkg2
 .....
 --sp-pkg8

Trusted world key pair is used for signing.

Going forward, this feature can be extended for Platfrom owned
Partitions, if required.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Ia6dfbc1447cfb41b1fcbd12cf2bf7b88f409bd8d