[device] Provide a driver-private data field for root devices

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/device.h

 	struct device dev;
 	/** Root device driver */
 	struct root_driver *driver;
+	/** Driver-private data */
+	void *priv;
 };
 
 /** A root device driver */
 /** Declare a root device */
 #define __root_device __table_entry ( ROOT_DEVICES, 01 )
 
+/**
+ * Set root device driver-private data
+ *
+ * @v rootdev		Root device
+ * @v priv		Private data
+ */
+static inline void rootdev_set_drvdata ( struct root_device *rootdev,
+					 void *priv ){
+	rootdev->priv = priv;
+}
+
+/**
+ * Get root device driver-private data
+ *
+ * @v rootdev		Root device
+ * @ret priv		Private data
+ */
+static inline void * rootdev_get_drvdata ( struct root_device *rootdev ) {
+	return rootdev->priv;
+}
+
 extern int device_keep_count;
 
 /**