dt-bindings: rtc: sun6i-rtc: Export internal RC oscillator

Experiments showed that on at least the H3/H5/A64 the RTC's internal
oscillator also feeds the CPUS mux in the PRCM.

Export this clock through the device tree, instead of having to use
a dummy fixed-clock device node, for the PRCM to consume. This will
properly describe the relationship between the clocks.

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