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>

include/button.h

@@ -0,0 +1,59 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (C) 2020 Philippe Reynes <philippe.reynes@softathome.com>
+ */
+
+#ifndef __BUTTON_H
+#define __BUTTON_H
+
+/**
+ * struct button_uc_plat - Platform data the uclass stores about each device
+ *
+ * @label:	Button label
+ */
+struct button_uc_plat {
+	const char *label;
+};
+
+/**
+ * enum button_state_t - State used for button
+ * - BUTTON_OFF - Button is not pressed
+ * - BUTTON_ON - Button is pressed
+ * - BUTTON_COUNT - Number of button state
+ */
+enum button_state_t {
+	BUTTON_OFF = 0,
+	BUTTON_ON = 1,
+	BUTTON_COUNT,
+};
+
+struct button_ops {
+	/**
+	 * get_state() - get the state of a button
+	 *
+	 * @dev:	button device to change
+	 * @return button state button_state_t, or -ve on error
+	 */
+	enum button_state_t (*get_state)(struct udevice *dev);
+};
+
+#define button_get_ops(dev)	((struct button_ops *)(dev)->driver->ops)
+
+/**
+ * button_get_by_label() - Find a button device by label
+ *
+ * @label:	button label to look up
+ * @devp:	Returns the associated device, if found
+ * @return 0 if found, -ENODEV if not found, other -ve on error
+ */
+int button_get_by_label(const char *label, struct udevice **devp);
+
+/**
+ * button_get_state() - get the state of a button
+ *
+ * @dev:	button device to change
+ * @return button state button_state_t, or -ve on error
+ */
+enum button_state_t button_get_state(struct udevice *dev);
+
+#endif