power: reset: ltc2952: use _optional variant of devm_gpiod_get

devm_gpiod_get_optional returns NULL if devm_gpiod_get would return an
ENOENT error pointer.

There is no semantic change intended.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Sebastian Reichel <sre@kernel.org>