iio: amplifiers: update license information

Use the new `SPDX-License-Identifier` tag to specify the license.
For ad8366, also update copyright years in the header part.
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/amplifiers/Kconfig

+# SPDX-License-Identifier: GPL-2.0
 #
 # Gain Amplifiers, etc.
 #

drivers/iio/amplifiers/ad8366.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * AD8366 SPI Dual-Digital Variable Gain Amplifier (VGA)
  *
- * Copyright 2012 Analog Devices Inc.
- *
- * Licensed under the GPL-2.
+ * Copyright 2012-2019 Analog Devices Inc.
  */
 
 #include <linux/device.h>