ARM: dts: mstar: Add rtc device node

This adds the definition of the rtc device node. The RTC being able to
work with the oscillator at 12Mhz for now, it shares the same xtal than
the watchdog.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
Signed-off-by: Daniel Palmer <daniel@0x0f.com>
Link: https://lore.kernel.org/all/20210823171613.18941-4-romain.perier@gmail.com