plat: fix switch statements to comply with MISRA rules

Ensure (where possible) that switch statements in plat comply with MISRA
rules 16.1 - 16.7.

Change-Id: Ie4a7d2fd10f6141c0cfb89317ea28a755391622f
Signed-off-by: Jonathan Wright <jonathan.wright@arm.com>
diff --git a/plat/mediatek/common/mtk_sip_svc.c b/plat/mediatek/common/mtk_sip_svc.c
index beb2a69..869a959 100644
--- a/plat/mediatek/common/mtk_sip_svc.c
+++ b/plat/mediatek/common/mtk_sip_svc.c
@@ -71,6 +71,9 @@
 			boot_to_kernel(x1, x2, x3, x4);
 			SMC_RET0(handle);
 #endif
+		default:
+			/* Do nothing in default case */
+			break;
 		}
 	}