test(ff-a): FFA_FEATURES returns max RXTX buffer size

Add testcase to check that a call to `FFA_FEATURES` with
`FFA_RXTX_MAP_SMC64` returns a parameter explaining the maximum and
minimum buffer size on v1.2 or greater.

Change-Id: I03efc89655daf9f7c2eb9cf35d26c5a68690036a
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
3 files changed