dt-bindings: rtc: Add Realtek RTD1295

Add a binding for the RTC on the Realtek RTD119x/RTD129x SoC families.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>