ARM: dts: sun8i: v3s: Fix the RTC node

For some reason, while the v3s has a dedicated compatible in the RTC
binding, the one actually used was the A31's. However, it turns out that
the controller is pretty different (which justified the compatible).

Let's use the proper compatible, and use the proper binding description as
well.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>