PSCI: Fix the return code for invalid entrypoint

As per PSCI1.0 specification, the error code to be returned when an invalid
non secure entrypoint address is specified by the PSCI client for CPU_SUSPEND,
CPU_ON or SYSTEM_SUSPEND must be PSCI_E_INVALID_ADDRESS. The current PSCI
implementation returned PSCI_E_INVAL_PARAMS. This patch rectifies this error
and also implements a common helper function to validate the entrypoint
information to be used across these PSCI API implementations.

Change-Id: I52d697d236c8bf0cd3297da4008c8e8c2399b170
diff --git a/include/bl31/services/psci.h b/include/bl31/services/psci.h
index c9b3f8d..f6fd487 100644
--- a/include/bl31/services/psci.h
+++ b/include/bl31/services/psci.h
@@ -165,6 +165,7 @@
 #define PSCI_E_INTERN_FAIL	-6
 #define PSCI_E_NOT_PRESENT	-7
 #define PSCI_E_DISABLED		-8
+#define PSCI_E_INVALID_ADDRESS	-9
 
 #define PSCI_INVALID_MPIDR	~(0ULL)