dt-bindings: rtc: sun6i: Add the R40 RTC compatible

The R40 has a pretty different RTC compared to the other SoCs we've
encountered so far, the most important difference being that it now has
only a single interrupt, compared to the previous SoCs having two.

Let's add a compatible for that.

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