dm: pci: Add a comment about how to find struct pci_controller

With driver mode, struct pci_controller is stored as uclass-private data.
Add a comment to that effect.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/include/pci.h b/include/pci.h
index 9c19482..c4f6577 100644
--- a/include/pci.h
+++ b/include/pci.h
@@ -537,6 +537,8 @@
 
 /*
  * Structure of a PCI controller (host bridge)
+ *
+ * With driver model this is dev_get_uclass_priv(bus)
  */
 struct pci_controller {
 #ifdef CONFIG_DM_PCI