PSCI: Add support for PSCI NODE_HW_STATE API

This patch adds support for NODE_HW_STATE PSCI API by introducing a new
PSCI platform hook (get_node_hw_state). The implementation validates
supplied arguments, and then invokes this platform-defined hook and
returns its result to the caller. PSCI capabilities are updated
accordingly.

Also updates porting and firmware design guides.

Change-Id: I808e55bdf0c157002a7c104b875779fe50a68a30
diff --git a/lib/psci/psci_setup.c b/lib/psci/psci_setup.c
index 20d0635..1ac1f23 100644
--- a/lib/psci/psci_setup.c
+++ b/lib/psci/psci_setup.c
@@ -256,6 +256,8 @@
 		psci_caps |=  define_psci_cap(PSCI_SYSTEM_OFF);
 	if (psci_plat_pm_ops->system_reset)
 		psci_caps |=  define_psci_cap(PSCI_SYSTEM_RESET);
+	if (psci_plat_pm_ops->get_node_hw_state)
+		psci_caps |= define_psci_cap(PSCI_NODE_HW_STATE_AARCH64);
 
 #if ENABLE_PSCI_STAT
 	psci_caps |=  define_psci_cap(PSCI_STAT_RESIDENCY_AARCH64);