cert_create: add Platform owned secure partitions support

Add support to generate a certificate named "plat-sp-cert" for Secure
Partitions(SP) owned by Platform.
Earlier a single certificate file "sip-sp-cert" was generated which
contained hash of all 8 SPs, with this change SPs are divided into
two categories viz "SiP owned" and "Plat owned" containing 4 SPs each.

Platform RoT key pair is used for signing.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I5bd493cfce4cf3fc14b87c8ed1045f633d0c92b6