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>