feat(tsp): add FF-A support to the TSP

This patch adds the FF-A programming model in the test
secure payload to ensure that it can be used to test
the following spec features.

1. SP initialisation on the primary and secondary cpus.
2. An event loop to receive direct requests and respond
   with direct responses.
3. Ability to receive messages that indicate power on
   and off of a cpu.
4. Ability to handle a secure interrupt.

Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Signed-off-by: Marc Bonnici <marc.bonnici@arm.com>
Signed-off-by: Shruti <shruti.gupta@arm.com>
Change-Id: I81cf744904d5cdc0b27862b5e4bc6f2cfe58a13a