button: gpio: add DM_GPIO dependency

The gpio-button driver depends on DM_GPIO, add it to Kconfig to avoid
build errors.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Stefan Roese <sr@denx.de>
diff --git a/drivers/button/Kconfig b/drivers/button/Kconfig
index 6db3c5e..8ce2de3 100644
--- a/drivers/button/Kconfig
+++ b/drivers/button/Kconfig
@@ -20,6 +20,7 @@
 config BUTTON_GPIO
 	bool "Button gpio"
 	depends on BUTTON
+	depends on DM_GPIO
 	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.