cactus: update sample manifest to SPCI Beta1 format

Notice, the dtb generation wrapper script had a provision
to automatically generate an "extra" memory description section.
This will be revisited later when SP boot time memory region
is implemented in SPM.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I4fb9e23004871d74eee4a1d1f4be0826695e3e9f