drivers/pps/clients/pps-gpio.c: convert to module_platform_driver

This removes some boilerplate code (no functional changes).

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>