cactus: update to SPCI Beta1 and add sample direct messaging test

The scenario is: TFTF (acting as an "hypervisor" at NS-EL2) sends a
direct message request to an SP targetting its SP ID. SPMD/SPMC relays
the message down to SP. SP extracts the message payload, amends it and
sends a direct message response to TFTF that SPMC/SPMD relay back to
TFTF. This sample SP can be instantiated into multiple SPs. The early
main entry flow discriminates the first VM from other secondary VMs by
getting its running id through SPCI_ID_GET. The SP runs an infinite
message loop and waits for a new message by calling SPCI_MSG_WAIT.
Notice SPCI_MSG_WAIT is also the hint to SPMD that the SP initialized
properly. About logging, only PVM for now has direct access to UART.
2nd VMs must use the debug log hypervisor call.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I001261c3f54e0bc217e43fca6812d6cda1ac4430