PSCI: Rework generic code to conform to coding guidelines

This patch reworks the PSCI generic implementation to conform to ARM
Trusted Firmware coding guidelines as described here:
https://github.com/ARM-software/arm-trusted-firmware/wiki

This patch also reviews the use of signed data types within PSCI
Generic code and replaces them with their unsigned counterparts wherever
they are not appropriate. The PSCI_INVALID_DATA macro which was defined
to -1 is now replaced with PSCI_INVALID_PWR_LVL macro which is defined
to PLAT_MAX_PWR_LVL + 1.

Change-Id: Iaea422d0e46fc314e0b173c2b4c16e0d56b2515a
diff --git a/services/std_svc/psci/psci_on.c b/services/std_svc/psci/psci_on.c
index d68198f..cf1a782 100644
--- a/services/std_svc/psci/psci_on.c
+++ b/services/std_svc/psci/psci_on.c
@@ -84,9 +84,9 @@
  * The state of all the relevant power domains are changed after calling the
  * platform handler as it can return error.
  ******************************************************************************/
-int psci_cpu_on_start(unsigned long target_cpu,
+int psci_cpu_on_start(u_register_t target_cpu,
 		      entry_point_info_t *ep,
-		      int end_pwrlvl)
+		      unsigned int end_pwrlvl)
 {
 	int rc;
 	unsigned int target_idx = plat_core_pos_by_mpidr(target_cpu);
@@ -130,7 +130,7 @@
 	 * of the target cpu to allow it to perform the necessary
 	 * steps to power on.
 	 */
-	rc = psci_plat_pm_ops->pwr_domain_on((u_register_t)target_cpu);
+	rc = psci_plat_pm_ops->pwr_domain_on(target_cpu);
 	assert(rc == PSCI_E_SUCCESS || rc == PSCI_E_INTERN_FAIL);
 
 	if (rc == PSCI_E_SUCCESS)