sandbox: Convert GPIOs to use driver model

Convert sandbox over to use driver model GPIOs.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/configs/sandbox.h b/include/configs/sandbox.h
index 1d11c40..6f424e1 100644
--- a/include/configs/sandbox.h
+++ b/include/configs/sandbox.h
@@ -26,6 +26,7 @@
 #define CONFIG_DM_DEMO
 #define CONFIG_DM_DEMO_SIMPLE
 #define CONFIG_DM_DEMO_SHAPE
+#define CONFIG_DM_GPIO
 #define CONFIG_DM_TEST
 
 /* Number of bits in a C 'long' on this architecture */