of: introduce of_property_read_s32

Introduce signed 32bit integer of_property_read method.
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Grant Likely <grant.likely@linaro.org>

include/linux/of.h

@@ -760,6 +760,13 @@ static inline int of_property_read_u32(const struct device_node *np,
 	return of_property_read_u32_array(np, propname, out_value, 1);
 }
 
+static inline int of_property_read_s32(const struct device_node *np,
+				       const char *propname,
+				       s32 *out_value)
+{
+	return of_property_read_u32(np, propname, (u32*) out_value);
+}
+
 #define of_property_for_each_u32(np, propname, prop, p, u)	\
 	for (prop = of_find_property(np, propname, NULL),	\
 		p = of_prop_next_u32(prop, NULL, &u);		\