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/interrupts.c b/src/ffa/spmc/interrupts.c
index 70cded3..9110b6f 100644
--- a/src/ffa/spmc/interrupts.c
+++ b/src/ffa/spmc/interrupts.c
@@ -105,7 +105,7 @@
 		 */
 		break;
 	case ARM_EL1_VIRT_TIMER_PHYS_INT:
-		/* Fall through */
+		[[fallthrough]];
 	case ARM_EL1_PHYS_TIMER_PHYS_INT:
 		panic("Timer interrupt not expected to fire: %u\n",
 		      interrupt_id);
@@ -468,6 +468,7 @@
 			 * Fall through in the case handling has not been fully
 			 * completed.
 			 */
+			[[fallthrough]];
 		default:
 			/*
 			 * Either invoke the handler related to partitions from