device-tree: add bus_size_cells

kernel/lib/device-tree/src/driver/tree.rs

@@ -406,6 +406,11 @@ impl Node {
         self.bus_address_cells
     }
 
+    /// Returns the `#size-cells` value of the node's parent bus
+    pub fn bus_size_cells(&self) -> usize {
+        self.bus_size_cells
+    }
+
     /// Returns the `#interrupt-cells` of this node, if any
     pub fn self_interrupt_cells(&self) -> Option<usize> {
         self.prop_usize("#interrupt-cells")