drivers: cpuidle: Add status property to ARM idle states

On some platforms the device tree bindings must provide the kernel
with a status flag for idle states, that defines whether the idle
state is operational or not in the current configuration.

This patch adds a status property to the ARM idle states compliant
with ePAPR v1.1 and updates the DT parsing code accordingly.

Acked-by: Kevin Hilman <[e-mail]>
Signed-off-by: Lorenzo Pieralisi <[e-mail]>
Signed-off-by: Daniel Lezcano <[e-mail]>

Documentation/devicetree/bindings/arm/idle-states.txt

···
 			    In such systems entry-latency-us + exit-latency-us
 			    will exceed wakeup-latency-us by this duration.
 
+	- status:
+		Usage: Optional
+		Value type: <string>
+		Definition: A standard device tree property [5] that indicates
+			    the operational status of an idle-state.
+			    If present, it shall be:
+			    "okay": to indicate that the idle state is
+				    operational.
+			    "disabled": to indicate that the idle state has
+					been disabled in firmware so it is not
+					operational.
+			    If the property is not present the idle-state must
+			    be considered operational.
+
 	In addition to the properties listed above, a state node may require
 	additional properties specifics to the entry-method defined in the
 	idle-states node, please refer to the entry-method bindings

drivers/cpuidle/dt_idle_states.c

···
 		if (!state_node)
 			break;
 
+		if (!of_device_is_available(state_node))
+			continue;
+
 		if (!idle_state_valid(state_node, i, cpumask)) {
 			pr_warn("%s idle state not valid, bailing out\n",
 				state_node->full_name);