cactus: handle 'CACTUS_REQ_ECHO_CMD' and 'CACTUS_ECHO_CMD'

Changing message loop to handle newly added commands.
The intent is to use these commands to validate FF-A direct messaging
between SPs.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: Id0ecbfafb13918fa2d349e70164b735f9ade6841
2 files changed