Tegra186: implement prepare_system_off handler

This patch issues the 'System Off' ARI to power off the entire
system from the 'prepare_system_off' handler.
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>

plat/nvidia/tegra/soc/t186/plat_psci_handlers.c

@@ -33,6 +33,7 @@
 #include <debug.h>
 #include <mce.h>
 #include <psci.h>
+#include <t18x_ari.h>
 #include <tegra_private.h>
 
 int32_t tegra_soc_validate_power_state(unsigned int power_state)
@@ -93,3 +94,8 @@ int tegra_soc_prepare_cpu_off(unsigned long mpidr)
 	/* Turn off CPU */
 	return mce_command_handler(MCE_CMD_ENTER_CSTATE, ~0, 0, 0);
 }
+
+__dead2 void tegra_soc_prepare_system_off(void)
+{
+	mce_enter_ccplex_state(TEGRA_ARI_MISC_CCPLEX_SHUTDOWN_POWER_OFF);
+}