rtc: pm8xxx: refactor read_time()

In preparation for adding support for setting the time by means of an
externally stored offset, refactor read_time() by adding a new helper
that can be used to retrieve the raw time as stored in the RTC.

Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20230202155448.6715-14-johan+linaro@kernel.org


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>