refactor(rse): put MHU code in a dedicated file

To be able to use RSE comms without MHU, a first step is to disentangle
the rse_comms.c file with MHU code direct calls. This is done with the
creation of a new file rse_comms_mhu.c. New APIs are created to
initialize the mailbox, get max message size and send and receive data.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I75dda77e1886beaa6ced6f92c311617125918cfa
diff --git a/include/drivers/arm/mhu.h b/include/drivers/arm/mhu.h
index 31c6a81..cc74e27 100644
--- a/include/drivers/arm/mhu.h
+++ b/include/drivers/arm/mhu.h
@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2022, Arm Limited. All rights reserved.
+ * Copyright (c) 2025, Arm Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -25,6 +26,14 @@
 };
 
 /**
+ * Structure used by RSE comms
+ */
+struct mhu_addr {
+	uintptr_t sender_base;
+	uintptr_t receiver_base;
+};
+
+/**
  * Initializes sender MHU.
  *
  * mhu_sender_base	Base address of sender MHU.