dt-bindings: ufs: cdns,ufshc: Add power-domains

The Cadence UFS controller can be part of power domain (as it is in
example DTS of TI J721e UFS Host Controller Glue), so allow such
property.

Reported-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220427065802.110402-1-krzysztof.kozlowski@linaro.org