dm: core: Convert driver_bind() to use const

The driver is not modified by driver model, so update driver_bind() to
recognise that.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>
diff --git a/include/dm/device.h b/include/dm/device.h
index f27b34b..fafecce 100644
--- a/include/dm/device.h
+++ b/include/dm/device.h
@@ -70,7 +70,7 @@
  * when the device is probed and will be unique within the device's uclass.
  */
 struct udevice {
-	struct driver *driver;
+	const struct driver *driver;
 	const char *name;
 	void *platdata;
 	void *parent_platdata;