fix: unify fallthrough annotations

Compiling with -Wimplicit-fallthrough=3 (enabled by -Wextra) produces
many warnings about fallthrough comments either missing or being wrong.
Unify the comments so we comply with -Wextra.

Note that Coverity recommends against using the __attribute__ directive.
Also, zlib does not build with a higher value of -Wimplicit-fallthrough.
Finally, compilers strip comments before expanding macros. As such,
checkpatch's fallthrough annotation (or higher levels of the flag) isn't
really possible.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I060cf4f8dc04c02cbb45cf4ceb69569a8369ccee
diff --git a/services/std_svc/spmd/spmd_main.c b/services/std_svc/spmd/spmd_main.c
index 7e6c89d..afd0f2e 100644
--- a/services/std_svc/spmd/spmd_main.c
+++ b/services/std_svc/spmd/spmd_main.c
@@ -868,7 +868,8 @@
 						     FFA_ERROR_NOT_SUPPORTED);
 		}
 
-		/* Fall through to forward the call to the other world */
+		/* Forward the call to the other world */
+		/* fallthrough */
 	case FFA_MSG_SEND:
 	case FFA_MSG_SEND_DIRECT_RESP_SMC64:
 	case FFA_MEM_DONATE_SMC32:
@@ -908,7 +909,8 @@
 			spmd_spm_core_sync_exit(0ULL);
 		}
 
-		/* Fall through to forward the call to the other world */
+		/* Forward the call to the other world */
+		/* fallthrough */
 	case FFA_INTERRUPT:
 	case FFA_MSG_YIELD:
 		/* This interface must be invoked only by the Secure world */