Add optional PSCI STAT residency & count functions

This patch adds following optional PSCI STAT functions:

- PSCI_STAT_RESIDENCY: This call returns the amount of time spent
  in power_state in microseconds, by the node represented by the
  `target_cpu` and the highest level of `power_state`.

- PSCI_STAT_COUNT: This call returns the number of times a
  `power_state` has been used by the node represented by the
  `target_cpu` and the highest power level of `power_state`.

These APIs provides residency statistics for power states that has
been used by the platform. They are implemented according to v1.0
of the PSCI specification.

By default this optional feature is disabled in the PSCI
implementation. To enable it, set the boolean flag
`ENABLE_PSCI_STAT` to 1. This also sets `ENABLE_PMF` to 1.

Change-Id: Ie62e9d37d6d416ccb1813acd7f616d1ddd3e8aff
diff --git a/bl31/bl31.mk b/bl31/bl31.mk
index 0491a1a..8a7fccb 100644
--- a/bl31/bl31.mk
+++ b/bl31/bl31.mk
@@ -63,6 +63,10 @@
 BL31_SOURCES		+=	lib/pmf/pmf_main.c
 endif
 
+ifeq (${ENABLE_PSCI_STAT}, 1)
+BL31_SOURCES		+=	services/std_svc/psci/psci_stat.c
+endif
+
 BL31_LINKERFILE		:=	bl31/bl31.ld.S
 
 # Flag used to indicate if Crash reporting via console should be included