dm: button: add an uclass for button

Add a new uclass for button that implements two functions:
- button_get_by_label
- button_get_status

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
diff --git a/drivers/Kconfig b/drivers/Kconfig
index 7a839fa..119e412 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -16,6 +16,8 @@
 
 source "drivers/bootcount/Kconfig"
 
+source "drivers/button/Kconfig"
+
 source "drivers/cache/Kconfig"
 
 source "drivers/clk/Kconfig"