of: introduce of_get_available_child_count

Some drivers keep counting available child by themselves. So
introduce a new simple API like of_get_child_count() but for
available childs.

Cc: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>

include/linux/of.h

@@ -226,6 +226,17 @@ static inline int of_get_child_count(const struct device_node *np)
 	return num;
 }
 
+static inline int of_get_available_child_count(const struct device_node *np)
+{
+	struct device_node *child;
+	int num = 0;
+
+	for_each_available_child_of_node(np, child)
+		num++;
+
+	return num;
+}
+
 extern struct device_node *of_find_node_with_property(
 	struct device_node *from, const char *prop_name);
 #define for_each_node_with_property(dn, prop_name) \
@@ -376,6 +387,11 @@ static inline int of_get_child_count(const struct device_node *np)
 	return 0;
 }
 
+static inline int of_get_available_child_count(const struct device_node *np)
+{
+	return 0;
+}
+
 static inline int of_device_is_compatible(const struct device_node *device,
 					  const char *name)
 {