cactus: adding memory sharing tests

Added tests to FF-A memory sharing interface. To enable them:
- The message loop was refactored to support multiple requests/commands
 coming through direct messaging, leveraging command wrappers
 from the previous commit.
 - Enabled adding dynamic memory regions to translation table, according
 to xlat_tables_v2 library documentation [1];
 - Mapped translation tables into cactus memory mapping so that they can
 be updated after a memory retrieve.

[1] https://trustedfirmware-a.readthedocs.io/en/latest/components/xlat-tables-lib-v2-design.html?highlight=xlat

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I73c9ba66dd89c53c50eca6ba6aca2e0ad115cb35
5 files changed