rtc: Add support for the MSTAR MSC313 RTC

This adds support for the RTC block on the Mstar MSC313e SoCs and newer.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Co-developed-by: Romain Perier <romain.perier@gmail.com>
Signed-off-by: Romain Perier <romain.perier@gmail.com>
Reviewed-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210823171613.18941-3-romain.perier@gmail.com