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/xilinx/zynqmp/aarch64/zynqmp_common.c b/plat/xilinx/zynqmp/aarch64/zynqmp_common.c
index d7a7d4e..fd054be 100644
--- a/plat/xilinx/zynqmp/aarch64/zynqmp_common.c
+++ b/plat/xilinx/zynqmp/aarch64/zynqmp_common.c
@@ -48,6 +48,9 @@
 		return 25000000;
 	case ZYNQMP_CSU_VERSION_QEMU:
 		return 133000000;
+	default:
+		/* Do nothing in default case */
+		break;
 	}
 
 	return 100000000;
@@ -187,6 +190,9 @@
 	case ZYNQMP_CSU_VERSION_SILICON:
 		label = "silicon";
 		break;
+	default:
+		/* Do nothing in default case */
+		break;
 	}
 
 	NOTICE("ATF running on XCZU%s/%s v%d/RTL%d.%d at 0x%x%s\n",
@@ -258,6 +264,9 @@
 		return 4000000;
 	case ZYNQMP_CSU_VERSION_QEMU:
 		return 50000000;
+	default:
+		/* Do nothing in default case */
+		break;
 	}
 
 	return mmio_read_32(IOU_SCNTRS_BASEFREQ);