dt: add helper to read 64-bit integers

Add a helper similar to of_property_read_u32() that handles 64-bit
integers.

v2/v3: constify device node and property name parameters.

Cc: Grant Likely <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Jamie Iles <[e-mail]>
Signed-off-by: Grant Likely <[e-mail]>

drivers/of/base.c

···
 EXPORT_SYMBOL_GPL(of_property_read_u32_array);
 
 /**
+ * of_property_read_u64 - Find and read a 64 bit integer from a property
+ * @np:		device node from which the property value is to be read.
+ * @propname:	name of the property to be searched.
+ * @out_value:	pointer to return value, modified only if return value is 0.
+ *
+ * Search for a property in a device node and read a 64-bit value from
+ * it. Returns 0 on success, -EINVAL if the property does not exist,
+ * -ENODATA if property does not have a value, and -EOVERFLOW if the
+ * property data isn't large enough.
+ *
+ * The out_value is modified only if a valid u64 value can be decoded.
+ */
+int of_property_read_u64(const struct device_node *np, const char *propname,
+			 u64 *out_value)
+{
+	struct property *prop = of_find_property(np, propname, NULL);
+
+	if (!prop)
+		return -EINVAL;
+	if (!prop->value)
+		return -ENODATA;
+	if (sizeof(*out_value) > prop->length)
+		return -EOVERFLOW;
+	*out_value = of_read_number(prop->value, 2);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(of_property_read_u64);
+
+/**
  * of_property_read_string - Find and read a string from a property
  * @np:		device node from which the property value is to be read.
  * @propname:	name of the property to be searched.

include/linux/of.h

···
 				      const char *propname,
 				      u32 *out_values,
 				      size_t sz);
+extern int of_property_read_u64(const struct device_node *np,
+				const char *propname, u64 *out_value);
 
 extern int of_property_read_string(struct device_node *np,
 				   const char *propname,
···
 				int *lenp)
 {
 	return NULL;
+}
+
+static inline int of_property_read_u64(const struct device_node *np,
+				       const char *propname, u64 *out_value)
+{
+	return -ENOSYS;
 }
 
 #define of_match_ptr(_ptr)	NULL