power: supply: generic-adc-battery: update copyright info

jz4740-battery.c and s3c_adc_battery.c have been removed
from the tree and after all of my restructuring the driver
is basically no longer based on them.

Thus update the copyright information and switch to SPDX
license identifier while being at it.
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>

drivers/power/supply/generic-adc-battery.c

+// SPDX-License-Identifier: GPL-2.0
 /*
- * Generic battery driver code using IIO
+ * Generic battery driver using IIO
  * Copyright (C) 2012, Anish Kumar <anish198519851985@gmail.com>
- * based on jz4740-battery.c
- * based on s3c_adc_battery.c
- *
- * This file is subject to the terms and conditions of the GNU General Public
- * License.  See the file COPYING in the main directory of this archive for
- * more details.
- *
+ * Copyright (c) 2023, Sebastian Reichel <sre@kernel.org>
  */
 #include <linux/interrupt.h>
 #include <linux/platform_device.h>