dm: button: add a driver for button driven by gpio

Add a simple driver which allows use of buttons attached to GPIOs.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
diff --git a/drivers/button/Kconfig b/drivers/button/Kconfig
index 8301858..6b3ec7e 100644
--- a/drivers/button/Kconfig
+++ b/drivers/button/Kconfig
@@ -9,4 +9,13 @@
 	  can provide access to board-specific buttons. Use of the device tree
 	  for configuration is encouraged.
 
+config BUTTON_GPIO
+	bool "Button gpio"
+	depends on BUTTON
+	help
+	  Enable support for buttons which are connected to GPIO lines. These
+	  GPIOs may be on the SoC or some other device which provides GPIOs.
+	  The GPIO driver must used driver model. Buttons are configured using
+	  the device tree.
+
 endmenu