Validate psci_cpu_on_start() arguments

This patch introduces some debug assertions in the function
psci_cpu_on_start() to check the arguments it receives are
valid.

Change-Id: If4d23c9f668fb46f2d18c5e2ed1929498cc6736b

services/std_svc/psci/psci_on.c

@@ -74,6 +74,10 @@ int psci_cpu_on_start(u_register_t target_cpu,
 	unsigned int target_idx = plat_core_pos_by_mpidr(target_cpu);
 	aff_info_state_t target_aff_state;
 
+	/* Calling function must supply valid input arguments */
+	assert((int) target_idx >= 0);
+	assert(ep != NULL);
+
 	/*
 	 * This function must only be called on platforms where the
 	 * CPU_ON platform hooks have been implemented.