dts: bindings: Rename i2c.yaml to i2c-controller.yaml

Renaming bindings to consistently be called foo-controller.yaml for
controllers and foo-device.yaml for devices (last one mostly makes sense
for devices on buses and the like).

I was thinking of having a plain foo.yaml be the controller as well, but

    !include interrupt.yaml

reads much worse than

    !include interrupt-controller.yaml

Another advantage of this approach is that no binding changes meaning
(which could be risky). It's just adding suffixes to filenames.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>