Remove several warnings reported with W=2

Improved support for W=2 compilation flag by solving some nested-extern
and sign-compare warnings.

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

Outside of libraries, some warnings cannot be fixed.

Change-Id: I06b1923857f2a6a50e93d62d0274915b268cef05
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
diff --git a/drivers/arm/css/scp/css_pm_scmi.c b/drivers/arm/css/scp/css_pm_scmi.c
index 1966c44..8dbefa1 100644
--- a/drivers/arm/css/scp/css_pm_scmi.c
+++ b/drivers/arm/css/scp/css_pm_scmi.c
@@ -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
  */
@@ -99,7 +99,7 @@
 		return;
 	}
 #if !HW_ASSISTED_COHERENCY
-	int lvl;
+	unsigned int lvl;
 	uint32_t scmi_pwr_state = 0;
 	/*
 	 * If we reach here, then assert that power down at system power domain
@@ -145,7 +145,8 @@
  */
 void css_scp_off(const struct psci_power_state *target_state)
 {
-	int lvl = 0, ret;
+	unsigned int lvl = 0;
+	int ret;
 	uint32_t scmi_pwr_state = 0;
 
 	/* At-least the CPU level should be specified to be OFF */
@@ -184,7 +185,8 @@
  */
 void css_scp_on(u_register_t mpidr)
 {
-	int lvl = 0, ret, core_pos;
+	unsigned int lvl = 0;
+	int ret, core_pos;
 	uint32_t scmi_pwr_state = 0;
 
 	for (; lvl <= PLAT_MAX_PWR_LVL; lvl++)