ARM: dts: sun8i: r40: Change the RTC compatible

Unlike what's being reported in the device tree so far, the RTC in the R40
is quite different from the H3. Indeed it doesn't have the internal
oscillator output, and it has only a single interrupt. Let's add a
compatible for it.

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