rtc: cros-ec: Make license text and module license match.

The license text is specifying "GPLv2" but the MODULE_LICENSE is set to
GPL which means GNU Public License v2 or later. When MODULE_LICENSE and
boiler plate does not match, go for boiler plate license.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>