dm: i2c: Add a note to I2C drivers which need conversion

Maintainers need to be notified more directly of the need to convert these
drivers. Add a note to the top each affected file.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heiko Schocher <hs@denx.de>
diff --git a/drivers/i2c/ppc4xx_i2c.c b/drivers/i2c/ppc4xx_i2c.c
index df88885..8a38d11 100644
--- a/drivers/i2c/ppc4xx_i2c.c
+++ b/drivers/i2c/ppc4xx_i2c.c
@@ -8,6 +8,9 @@
  * Bill Hunter,  Wave 7 Optics, williamhunter@mediaone.net
  *
  * SPDX-License-Identifier:	GPL-2.0+
+ *
+ * NOTE: This driver should be converted to driver model before June 2017.
+ * Please see doc/driver-model/i2c-howto.txt for instructions.
  */
 
 #include <common.h>