Document plat_psci_ops.(system_off|system_reset)

fixes ARM-Software/tf-issues#346

Change-Id: I946c8eab650bba7407775462ce91f394d767e079
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>

docs/porting-guide.md

@@ -1834,6 +1834,18 @@ The `target_state` (first argument) has a similar meaning as described in
 the `pwr_domain_on_finish()` operation. The generic code expects the platform
 to succeed.
 
+#### plat_psci_ops.system_off()
+
+This function is called by PSCI implementation in response to a `SYSTEM_OFF`
+call. It performs the platform-specific system poweroff sequence after
+notifying the Secure Payload Dispatcher.
+
+#### plat_psci_ops.system_reset()
+
+This function is called by PSCI implementation in response to a `SYSTEM_RESET`
+call. It performs the platform-specific system reset sequence after
+notifying the Secure Payload Dispatcher.
+
 #### plat_psci_ops.validate_power_state()
 
 This function is called by the PSCI implementation during the `CPU_SUSPEND`