dtoc: Track nodes which are actually used

Mark all nodes that are actually used, so we can perform extra checks on
them.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/test_src_scan.py b/tools/dtoc/test_src_scan.py
index a7eba30..ebdc12a 100644
--- a/tools/dtoc/test_src_scan.py
+++ b/tools/dtoc/test_src_scan.py
@@ -98,7 +98,7 @@
         drv3.uclass_id = i2c
         drv3.compat = compat
         self.assertEqual(
-            "Driver(name='fred', uclass_id='I2C_UCLASS', "
+            "Driver(name='fred', used=False, uclass_id='I2C_UCLASS', "
             "compat={'rockchip,rk3288-grf': 'ROCKCHIP_SYSCON_GRF', "
             "'rockchip,rk3288-srf': None}, priv=)", str(drv1))
         self.assertEqual(drv1, drv3)