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_off.c b/services/std_svc/psci/psci_off.c
index 28fa52c..f565ffb 100644
--- a/services/std_svc/psci/psci_off.c
+++ b/services/std_svc/psci/psci_off.c
@@ -60,7 +60,7 @@
  * interconnect level if the cpu is the last in the cluster and also the
  * program the power controller.
  ******************************************************************************/
-int psci_do_cpu_off(int end_pwrlvl)
+int psci_do_cpu_off(unsigned int end_pwrlvl)
 {
 	int rc, idx = plat_my_core_pos();
 	psci_power_state_t state_info;