refactor(ffa): remove `plat` prefix

Rename files to remove the `plat` prefix and replace with a
module-specific prefix.

Change-Id: Ie64cefcdf91da7b20e520828d8e234af12ab5c85
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/src/hf_ipi.c b/src/hf_ipi.c
index 0964c13..568efde 100644
--- a/src/hf_ipi.c
+++ b/src/hf_ipi.c
@@ -85,7 +85,7 @@
 
 	switch (target_vcpu->state) {
 	case VCPU_STATE_WAITING:
-		plat_ffa_sri_trigger_not_delayed(target_vcpu->cpu);
+		ffa_notifications_sri_trigger_not_delayed(target_vcpu->cpu);
 		return true;
 	case VCPU_STATE_RUNNING:
 	case VCPU_STATE_BLOCKED: