Remove several warnings reported with W=1

Improved support for W=1 compilation flag by solving missing-prototypes
and old-style-definition warnings.

The libraries are compiling with warnings (which turn into errors with
the Werror flag).

Outside of libraries, some warnings cannot be fixed without heavy
structural changes.

Change-Id: I1668cf99123ac4195c2a6a1d48945f7a64c67f16
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
diff --git a/include/drivers/arm/css/scmi.h b/include/drivers/arm/css/scmi.h
index df259f7..1f8dc6c 100644
--- a/include/drivers/arm/css/scmi.h
+++ b/include/drivers/arm/css/scmi.h
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017-2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2019, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -162,7 +162,7 @@
 int scmi_ap_core_get_reset_addr(void *p, uint64_t *reset_addr, uint32_t *attr);
 
 /* API to get the platform specific SCMI channel information. */
-scmi_channel_plat_info_t *plat_css_get_scmi_info();
+scmi_channel_plat_info_t *plat_css_get_scmi_info(void);
 
 /* API to override default PSCI callbacks for platforms that support SCMI. */
 const plat_psci_ops_t *css_scmi_override_pm_ops(plat_psci_ops_t *ops);