dt-bindings: rtc: sun6i: Clean up repetition

- Use "enum" for compatibles instead of several "const" alternatives.
- Merge the H6 clock-output-names minItems/maxItems constraint into the
  identical block above.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220203021736.13434-2-samuel@sholland.org