dm: core: Add ofnode function to read a 64-bit int

We have a 32-bit version of this function. Add a 64-bit version as well so
we can easily read 64-bit ints from the device tree.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/dm/of_access.h b/include/dm/of_access.h
index 74f0606..dd1abb8 100644
--- a/include/dm/of_access.h
+++ b/include/dm/of_access.h
@@ -219,6 +219,22 @@
 int of_read_u32(const struct device_node *np, const char *propname, u32 *outp);
 
 /**
+ * of_read_u64() - Find and read a 64-bit integer from a property
+ *
+ * Search for a property in a device node and read a 64-bit value from
+ * it.
+ *
+ * @np:		device node from which the property value is to be read.
+ * @propname:	name of the property to be searched.
+ * @outp:	pointer to return value, modified only if return value is 0.
+ *
+ * @return 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.
+ */
+int of_read_u64(const struct device_node *np, const char *propname, u64 *outp);
+
+/**
  * of_read_u32_array() - Find and read an array of 32 bit integers
  *
  * Search for a property in a device node and read 32-bit value(s) from