rtc: bd70528: add BD71828 support

ROHM BD71828 PMIC RTC block is from many parts similar to one
on BD70528. Support BD71828 RTC using BD70528 RTC driver and
avoid re-inventing the wheel.

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>