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/button/Makefile b/drivers/button/Makefile
new file mode 100644
index 0000000..0b4c128
--- /dev/null
+++ b/drivers/button/Makefile
@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0+
+#
+# Copyright (C) 2020 Philippe Reynes <philippe.reynes@softathome.com>
+
+obj-$(CONFIG_BUTTON) += button-uclass.o