SPMD: loading Secure Partition payloads

This patch implements loading of Secure Partition packages using
existing framework of loading other bl images.

The current framework uses a statically defined array to store all the
possible image types and at run time generates a link list and traverse
through it to load different images.

To load SPs, a new array of fixed size is introduced which will be
dynamically populated based on number of SPs available in the system
and it will be appended to the loadable images list.

Change-Id: I8309f63595f2a71b28a73b922d20ccba9c4f6ae4
Signed-off-by: Manish Pandey <manish.pandey2@arm.com>