Ensure the flow through switch statements is clear

Ensure case clauses:
*   Terminate with an unconditional break, return or goto statement.
*   Use conditional break, return or goto statements as long as the end
    of the case clause is unreachable; such case clauses must terminate
    with assert(0) /* Unreachable */ or an unconditional  __dead2 function
    call
*   Only fallthough when doing otherwise would result in less
    readable/maintainable code; such case clauses must terminate with a
    /* Fallthrough */ comment to make it clear this is the case and
    indicate that a fallthrough is intended.

This reduces the chance of bugs appearing due to unintended flow through a
switch statement

Change-Id: I70fc2d1f4fd679042397dec12fd1982976646168
Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>
diff --git a/plat/rockchip/rk3399/drivers/dp/cdn_dp.c b/plat/rockchip/rk3399/drivers/dp/cdn_dp.c
index 2adab8f..73a75bb 100644
--- a/plat/rockchip/rk3399/drivers/dp/cdn_dp.c
+++ b/plat/rockchip/rk3399/drivers/dp/cdn_dp.c
@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
+#include <assert.h>
 #include <cdefs.h>
 #include <cdn_dp.h>
 #include <smccc.h>
@@ -38,6 +39,7 @@
 			return hdcp_handler(&key);
 		else
 			return PSCI_E_INVALID_PARAMS;
+		assert(0); /* Unreachable */
 	default:
 		return SMC_UNK;
 	}