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_helpers.S b/services/std_svc/psci/psci_helpers.S
index bbfa5d5..6ccf943 100644
--- a/services/std_svc/psci/psci_helpers.S
+++ b/services/std_svc/psci/psci_helpers.S
@@ -37,7 +37,7 @@
 	.globl	psci_do_pwrup_cache_maintenance
 
 /* -----------------------------------------------------------------------
- * void psci_do_pwrdown_cache_maintenance(uint32_t power level);
+ * void psci_do_pwrdown_cache_maintenance(unsigned int power level);
  *
  * This function performs cache maintenance for the specified power
  * level. The levels of cache affected are determined by the power
@@ -66,7 +66,7 @@
 	 * platform.
 	 * ---------------------------------------------
 	 */
-	cmp	x0, #PSCI_CPU_PWR_LVL
+	cmp	w0, #PSCI_CPU_PWR_LVL
 	b.eq	do_core_pwr_dwn
 	bl	prepare_cluster_pwr_dwn
 	b	do_stack_maintenance