ASoC: rockchip-i2s: add power-domains property

In the old txt situation we add/describe only properties that are used
by the driver/hardware itself. With yaml it also filters things in a
node that are used by other drivers like 'power-domains' for rk3399,
so add it to 'rockchip-i2s.yaml'.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200324094149.6904-3-jbx6244@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>