dt-bindings: rtc: sun6i-rtc: Rewrite clock outputs as a list

The clock output section for this binding describes the two outputs in
the descriptions for both the #clock-cells and clock-output-names
properties.

Instead of overlapping information that is hard to read, rewrite the
clock outputs as a list of indices and descriptions. The properies
can reference this list instead. This will also make it easier to
add notes or conditions to the clocks, and also for adding new outputs.

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