mfd: Add basic device tree binding for wm8994

Add a placeholder device tree binding for the wm8994 driver. At present
the binding is essentially null as none of the platform data is supported,
and at least some of that will depend on the pending regulator bindings.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
diff --git a/drivers/mfd/wm8994-core.c b/drivers/mfd/wm8994-core.c
index 5d6ba13..74d4746 100644
--- a/drivers/mfd/wm8994-core.c
+++ b/drivers/mfd/wm8994-core.c
@@ -581,6 +581,14 @@
 	kfree(wm8994);
 }
 
+static const struct of_device_id wm8994_of_match[] = {
+	{ .compatible = "wlf,wm1811", },
+	{ .compatible = "wlf,wm8994", },
+	{ .compatible = "wlf,wm8958", },
+	{ }
+};
+MODULE_DEVICE_TABLE(of, wm8994_of_match);
+
 static int wm8994_i2c_probe(struct i2c_client *i2c,
 			    const struct i2c_device_id *id)
 {
@@ -633,6 +641,7 @@
 		.name = "wm8994",
 		.owner = THIS_MODULE,
 		.pm = &wm8994_pm_ops,
+		.of_match_table = wm8994_of_match,
 	},
 	.probe = wm8994_i2c_probe,
 	.remove = wm8994_i2c_remove,