Add Trusted OS extra image parsing support for ARM standard platforms ...

Trusted OS may have extra images to be loaded. Load them one by one
and do the parsing. In this patch, ARM TF need to load up to 3 images
for optee os: header, pager and paged images. Header image is the info
about optee os and images. Pager image include pager code and data.
Paged image include the paging parts using virtual memory.

Change-Id: Ia3bcfa6d8a3ed7850deb5729654daca7b00be394
Signed-off-by: Summer Qin <summer.qin@arm.com>