dt-bindings: i2c: convert rockchip i2c bindings to yaml

Current dts files with 'i2c' nodes are manually verified.
In order to automate this process i2c-rk3x.txt
has to be converted to yaml. In the new setup
i2c-rk3x.yaml will inherit properties from
i2c-controller.yaml.

Also change document name in MAINTAINERS.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>