libsp: Add sp_rxtx.h mock implementation

Enabling the validation of SP layer RXTX calls from other SP layer
modules or from user code.

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: Ia08d5fc0e07a0f18a29c2cc3d8b30c63286954a8
4 files changed