dm: input: add button_kbd driver

Bootmenu requires an input device with arrows and enter key.
A common smartphone luckily has power, volume up/down buttons,
which may be used for controlling bootmenu.
To use driver, add 'button-kbd' to stdin.

Signed-off-by: Dzmitry Sankouski <dsankouski@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/input/Kconfig b/drivers/input/Kconfig
index 1c534be..32360d9 100644
--- a/drivers/input/Kconfig
+++ b/drivers/input/Kconfig
@@ -46,6 +46,15 @@
 	  laptops based on Apple SoCs. These keyboards use an
 	  Apple-specific HID-over-SPI protocol.
 
+config BUTTON_KEYBOARD
+	bool "Buttons as keyboard"
+	depends on BUTTON_GPIO
+	depends on DM_KEYBOARD
+	help
+	  Enable support for mapping buttons to keycode events. Use linux,code button driver
+	  dt node to define button-event mapping.
+	  For example, an arrows and enter may be implemented to navigate boot menu.
+
 config CROS_EC_KEYB
 	bool "Enable Chrome OS EC keyboard support"
 	depends on INPUT