Use the fallthrough statement in switch loops

Clang does not seem to work with 'fall through' in comments.

Change-Id: Idcbe373be33ef7247548f856bfaba7ceb7f749b5
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51498
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>

src/lib/prog_loaders.c

@@ -167,7 +167,8 @@ void payload_load(void)
 		if (CONFIG(PAYLOAD_FIT_SUPPORT)) {
 			fit_payload(payload, mapping);
 			break;
-		} /* else fall-through */
+		}
+		__fallthrough;
 	default:
 		die_with_post_code(POST_INVALID_ROM,
 				   "Unsupported payload type %d.\n", payload->cbfs_type);