rtc: ds1742: don't explicitly specify word_size and stride of nvmem

nvmem_register() assumes these values to be 1 if unset, so they don't
need to be set explicitly.

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