Convert the i2c-jz4780.txt file to ingenic,i2c.yaml.

Two things were changed in the process:
- the clock-frequency property can now only be set to the two values
  that can be set by the hardware;
- the dmas and dma-names properties are now required.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
[robh: add ref to i2c-controller.yaml]
Signed-off-by: Rob Herring <robh@kernel.org>