fix(ff-a): harden ffa_secondary_ep_register abi

Add more verbose comments with references to the specification for the
FFA_SECONDARY_EP_REGISTER ABI implementation.
Add FFA_FEATURES response to FFA_SECONDARY_EP_REGISTER ABI
for the SPMC case.
Add check whether ABI is called in context of Hypervisor or SPMC.
Reject the call in the former case.
Reject the FFA_SECONDARY_EP_REGISTER call for UP partitions.
Reject the FFA_SECONDARY_EP_REGISTER call after partition has
initialized.

Change-Id: I3ce723fc6cdde6f27709b664abfe2c7929e9d9b2
Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
diff --git a/src/arch/fake/hypervisor/ffa.c b/src/arch/fake/hypervisor/ffa.c
index 916aea8..9300963 100644
--- a/src/arch/fake/hypervisor/ffa.c
+++ b/src/arch/fake/hypervisor/ffa.c
@@ -328,3 +328,8 @@
 	(void)partitions;
 	(void)ret_count;
 }
+
+bool plat_ffa_is_secondary_ep_register_supported(void)
+{
+	return false;
+}