rtc: bd70528: Do not require parent data

The ROHM BD71828 and BD71815 RTC drivers only need the regmap
pointer from parent. Regmap can be obtained via dev_get_regmap()
so do not require parent to populate driver data for that.

BD70528 on the other hand requires parent data to access the
watchdog so leave the parent data for BD70528 here for now.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>