SPM: Merge 'internal_msg' members into handle

'internal_msg' is a member of handle, defining a dedicated type for
it causes troubles when:

- Defining interfaces for processing connections. Taking the handle
  as parameter makes message abstraction unnecessary, and taking
  message as paramter makes extra message-to-handle mapping happen.

This patch is basically a search and replace, which eases the review.
After the merging, several unnecessary mapping functions are removed
(set/get rhandle e.g.). Followed up patches focus on detailed refine.

Change-Id: I2ec1375fde470d4a3983848afb46575d3cb0b392
Signed-off-by: Ken Liu <Ken.Liu@arm.com>
diff --git a/secure_fw/spm/include/ffm/backend.h b/secure_fw/spm/include/ffm/backend.h
index 5efeac0..ecd52cb 100644
--- a/secure_fw/spm/include/ffm/backend.h
+++ b/secure_fw/spm/include/ffm/backend.h
@@ -33,13 +33,13 @@
 
     /* Runtime model-specific message handling mechanism. */
     psa_status_t (*messaging)(struct service_t *p_serv,
-                              struct tfm_msg_body_t *p_msg);
+                              struct conn_handle_t *hdl);
 
     /*
      * Runtime model-specific message replying.
      * Return the connection handle or the acked status code.
      */
-    int32_t (*replying)(struct tfm_msg_body_t *p_msg, int32_t status);
+    int32_t (*replying)(struct conn_handle_t *hdl, int32_t status);
 };
 
 /* RUNTIME MODEL BACKENDS DECLARATION */