Tegra: enable 'signed-comparison' compilation warning/errors

This patch enables the 'sign-compare' flag, to enable warning/errors
for comparisons between signed/unsigned variables. The warning has
been enabled for all the Tegra platforms, to start with.

Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>
diff --git a/plat/nvidia/tegra/common/tegra_pm.c b/plat/nvidia/tegra/common/tegra_pm.c
index 9445019..6f841ef 100644
--- a/plat/nvidia/tegra/common/tegra_pm.c
+++ b/plat/nvidia/tegra/common/tegra_pm.c
@@ -107,7 +107,7 @@
 void tegra_get_sys_suspend_power_state(psci_power_state_t *req_state)
 {
 	/* all affinities use system suspend state id */
-	for (int i = MPIDR_AFFLVL0; i <= PLAT_MAX_PWR_LVL; i++)
+	for (uint32_t i = MPIDR_AFFLVL0; i <= PLAT_MAX_PWR_LVL; i++)
 		req_state->pwr_domain_state[i] = PSTATE_ID_SOC_POWERDN;
 }