dt-bindings: i2c: nvidia,tegra20-i2c: define power-domains top-level

Properties are expected to be defined in top-level "properties:" block
and further customized in "if:then:".  Only one variant has power
domains, so add respective top-level property and disallow it for other
devices.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>