gpio: Clarify that <linux/gpio.h> is legacy

It should be clear to developers that they should not include
this file in new code.
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

include/linux/gpio.h

 /* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * <linux/gpio.h>
+ *
+ * This is the LEGACY GPIO bulk include file, including legacy APIs. It is
+ * used for GPIO drivers still referencing the global GPIO numberspace,
+ * and should not be included in new code.
+ *
+ * If you're implementing a GPIO driver, only include <linux/gpio/driver.h>
+ * If you're implementing a GPIO consumer, only include <linux/gpio/consumer.h>
+ */
 #ifndef __LINUX_GPIO_H
 #define __LINUX_GPIO_H