spi: rockchip: add power-domains property

The Rockchip rk3399 TRM mentions that pd_sdioaudio includes
sdio, spi, i2s and spdif. Add a power-domains property to
to match reality with spi-rockchip.yaml.
Signed-off-by: NJohan Jonker <jbx6244@gmail.com>
Acked-by: NHeiko Stuebner <heiko@sntech.de>
Acked-by: NKrzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/12af4fe6-4d35-cb4a-f5f6-06e3aba990cb@gmail.comSigned-off-by: NMark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/spi/spi-rockchip.yaml

@@ -82,6 +82,9 @@ properties:
       where the "sleep" configuration may describe the state
       the pins should be in during system suspend.
 
+  power-domains:
+    maxItems: 1
+
 required:
   - compatible
   - reg