fix(ff-a): tftf handling of its RXTX buffer pair

The way tftf was handling the RXTX buffer pair created a dependency
on a set of tests from 'test_ffa_setup_and_discovery.c'. This was
problematic for test configurations for which the SPM tests are
not present.

This patch removes such dependency:
- Delete the 'INIT_MAILBOX' macro, and 'init_mailbox' function;
- RXTX buffer pair allocated within the 'get_tftf_mailbox'.
They are mapped into the SPMC via FFA_RXTX_MAP, and are returned
in the function's argument.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: Ia010ebd21f11ab7ca6582b574ffc9179693b1eed