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/Makefile b/drivers/Makefile
index afd159e..2178871 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -1,5 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0+
 
+obj-$(CONFIG_$(SPL_TPL_)BUTTON) += button/
 obj-$(CONFIG_$(SPL_TPL_)CACHE) += cache/
 obj-$(CONFIG_$(SPL_TPL_)CLK) += clk/
 obj-$(CONFIG_$(SPL_TPL_)DM) += core/