refactor: warn against implicit fallthrough

Enable the `-Wimplicit-fallthrough` warning, which will warn if `case`
statements in a `switch` block fall through to the next `case`.
Intentional fallthrough can be signalled with the `[[fallthrough]]`
attribute which was added in C23.

Change-Id: I421cacadafba6457847c3a067483c97fdd9896cf
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
diff --git a/src/ffa/spmc/cpu_cycles.c b/src/ffa/spmc/cpu_cycles.c
index fd98edf..0b5809b 100644
--- a/src/ffa/spmc/cpu_cycles.c
+++ b/src/ffa/spmc/cpu_cycles.c
@@ -542,7 +542,7 @@
 	case FFA_MSG_SEND_DIRECT_REQ_64:
 	case FFA_MSG_SEND_DIRECT_REQ_32:
 	case FFA_MSG_SEND_DIRECT_REQ2_64:
-		/* Fall through. */
+		[[fallthrough]];
 	case FFA_RUN_32: {
 		/* Rules 1,2 section 7.2 EAC0 spec. */
 		if (ffa_direct_msg_precedes_in_call_chain(current_locked,
@@ -603,7 +603,7 @@
 	case FFA_MSG_SEND_DIRECT_REQ_64:
 	case FFA_MSG_SEND_DIRECT_REQ_32:
 	case FFA_MSG_SEND_DIRECT_REQ2_64:
-		/* Fall through. */
+		[[fallthrough]];
 	case FFA_RUN_32: {
 		/* Rules 1,2. */
 		if (ffa_direct_msg_precedes_in_call_chain(current_locked,
@@ -615,8 +615,8 @@
 		return true;
 	}
 	case FFA_MSG_SEND_DIRECT_RESP_64:
-	case FFA_MSG_SEND_DIRECT_RESP_32: {
-	case FFA_MSG_SEND_DIRECT_RESP2_64:
+	case FFA_MSG_SEND_DIRECT_RESP_32:
+	case FFA_MSG_SEND_DIRECT_RESP2_64: {
 		/* Rule 3. */
 		if (current_locked.vcpu->direct_request_origin.vm_id ==
 		    receiver_vm_id) {