dm: add entries to Kconfig

Create entries of CONFIG_DM, CONFIG_DM_SERIAL, CONFIG_DM_GPIO
and CONFIG_DM_SPI.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/gpio/Kconfig b/drivers/gpio/Kconfig
index e69de29..d21302f 100644
--- a/drivers/gpio/Kconfig
+++ b/drivers/gpio/Kconfig
@@ -0,0 +1,6 @@
+config DM_GPIO
+	bool "Enable Driver Model for GPIO drivers"
+	depends on DM
+	help
+	  If you want to use driver model for GPIO drivers, say Y.
+	  To use legacy GPIO drivers, say N.