fix(scmi): make msg_header variable volatile

When tf-a is built with the llvm toolchain, the compiler optimizes the
variable in some way which results in SCMI_MSG_GET_TOKEN assert failure
in multiple places. This patch makes the msg_header variable in
mailbox_mem struct volatile, which seems to fix the issue.

Signed-off-by: sahil <sahil@arm.com>
Change-Id: I1d28b30a6472fc23ed85adff9cf73dfb536e39cc

drivers/arm/css/scmi/scmi_private.h

@@ -136,7 +136,7 @@ typedef struct mailbox_mem {
 	uint64_t res_b; /* Reserved */
 	uint32_t flags;
 	volatile uint32_t len;
-	uint32_t msg_header;
+	volatile uint32_t msg_header;
 	uint32_t payload[];
 } mailbox_mem_t;