rtc: pcf2127: make module license match the file header

The header of the pcf2127 driver specifies GPL v2 only as license, so
use "GPL v2" as module license specifier instead of "GPL" as the latter
means "GNU Public License v2 or later".

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>