rtc: spacemit: support the SpacemiT P1 RTC

Add support for the RTC found in the SpacemiT P1 PMIC.  Initially
only setting and reading the time are supported.

The PMIC is implemented as a multi-function device.  This RTC is
probed based on this driver being named in a MFD cell in the simple
MFD I2C driver.

Signed-off-by: Alex Elder <elder@riscstar.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20250825172057.163883-5-elder@riscstar.com


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