rtc: Add support for the SSD202D RTC

rtc: Add support for the SSD202D RTC

Newer SigmaStar SSD202D SoCs contain a Real Time Clock, capable of
running while the system is sleeping (battery powered), this is not the
case with the other RTC on older SoCs. This adds basic support for this
RTC block.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
Co-developed-by: Daniel Palmer <daniel@0x0f.com>
Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Link: https://lore.kernel.org/r/20230913151606.69494-2-romain.perier@gmail.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>