SPM: RXTX map test on TFTF

Some FF-A interfaces use RXTX buffers to exchange information with SPMC.
To avoid repetition of RXTX mapping across the spm-related tests, and
prevent allocation of multiple pages for RXTX buffers within TFTF
runtime:
- Implemented test helpers that hold address of RXTX buffers;
- Implemented test to FFA_RXTX_MAP ABI, that also sets value of RXTX
buffers;
- Cleaned up memory sharing tests that previously implemented RXTX
mapping.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I4a67982d3d185bf83809156e4fce03c6edb967d9
6 files changed