button: add a simple Analog to Digital Converter device based button driver

Add a simple Analog to Digital Converter device based button driver. This
driver binds to the 'adc-keys' device tree node.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
diff --git a/drivers/button/Kconfig b/drivers/button/Kconfig
index 6b3ec7e..6db3c5e 100644
--- a/drivers/button/Kconfig
+++ b/drivers/button/Kconfig
@@ -9,6 +9,14 @@
 	  can provide access to board-specific buttons. Use of the device tree
 	  for configuration is encouraged.
 
+config BUTTON_ADC
+	bool "Button adc"
+	depends on BUTTON
+	help
+	  Enable support for buttons which are connected to Analog to Digital
+	  Converter device. The ADC driver must use driver model. Buttons are
+	  configured using the device tree.
+
 config BUTTON_GPIO
 	bool "Button gpio"
 	depends on BUTTON