Convert the jz4740-rtc doc to YAML, and update it to reflect the new
changes in the driver:
- More compatible strings are specified, with fallbacks if needed,
- The vendor-specific properties are now properly prefixed with the
  'ingenic,' prefix.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200311182318.22154-3-paul@crapouillou.netSigned-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>