dm: Add Kconfig for driver/demo

Add a suitable Kconfig for this directory.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/drivers/Kconfig b/drivers/Kconfig
index 128736d..021293e 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -2,6 +2,8 @@
 
 source "drivers/core/Kconfig"
 
+source "drivers/demo/Kconfig"
+
 source "drivers/pci/Kconfig"
 
 source "drivers/pcmcia/Kconfig"