rtc: bd70528: Support RTC on ROHM BD71815

BD71815 contains similar RTC block as BD71828. Only the address offsets
seem different. Support also BD71815 RTC using rtc-bd70528.

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>