TFTF: Refactor FF-A memory sharing tests

- Created helper functions to initialize ffa_memory_region and to send
the respective memory region to the SP, making it possible
to reuse the logic in SP-to-SP memory share tests.
- Added comments to document relevant aspects about memory sharing.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: Id3c6db092d5b5eeed746a5fae30b8c853876fc53
3 files changed